drivers: irqsteer: adjust CHn_MASK index computation

[dbaluta]

For IRQ_STEER, lower CHn_MASK register indexes are used to mask higher interrupt IDs. For instance, in the case of i.MX8MP, the mapping is as follows:

CHn_MASK4 => masks interrupts [31:0]
CHn_MASK3 => masks interrupts [63:32]
CHn_MASK2 => masks interrupts [95:64]
CHn_MASK1 => masks interrupts [127:96]
CHn_MASK0 => masks interrupts [159:128]

The IRQSTEER_GetRegIndex() function is used to fetch the CHn_MASK register index based on a given slice. The term "slice" is used to refer to an index of a CHn_MASK register in the set of CHn_MASK registers assigned to a certain master. Assuming the following partition scheme (i.MX8MP):

{ CHn_MASK4 } is assigned to MASTER0
{ CHn_MASK3, CHn_MASK2 } is assigned to MASTER1
{ CHn_MASK1, CHn_MASK0 } is assiged to MASTER2

CHn_MASK3 would be at slice (index) 0, CHn_MASK2 would be at slice 1, CHn_MASK1 would be at slice 0 and so on.

To compute the CHn_MASK register index found at a given slice, IRQSTEER_GetRegIndex() uses a base index, which is either the lowest or the highest CHn_MASK register index in a master's partition (for instance, for MASTER1 the higher would be 3 and the lowest would be 2).

For IRQ_STEER instances with an uneven number of CHn_MASK registers the base is the lowest index, while in the case of instances with an even number of CHn_MASK registers it's exactly the opposite. This is an issue because the software using this function might expect the base to either be the lowest or the highest index ALL THE TIME (since this affects the order in which the CHn_MASK register indexes are returned).

As such, fix this problem by making the base the highest CHn_MASK register index.

Prerequisites

• I have checked latest main branch and the issue still exists.
• I did not see it is stated as known-issue in release notes.
• No similar GitHub issue is related to this change.
• My code follows the commit guidelines of this project.
• I have performed a self-review of my own code.
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.

Describe the pull request

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Tests

• Test configuration (please complete the following information):
  • Hardware setting: i.MX8MP/i.MX8QXP/i.MX8QM
  • Toolchain:
  • Test Tool preparation:
  • Any other dependencies:
• Test executed
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