imxrt-flash: add port and bus mask enums

[gerard5]

Description

Adds enums common to the plo implementation, so that it can be used in per project board_config.h instead of using magic numbers.

e.g.:
#define FLEXSPI2_PORT_MASK (flexspi_slBusA1 | flexspi_slBusA2 | flexspi_slBusB2)
instead of magic numbers:
#define FLEXSPI2_PORT_MASK (1 | 2 | 8)

JIRA: NIL-539

Motivation and Context

Types of changes

• 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 change)
• Chore (refactoring, style fixes, git/CI config, submodule management, no code logic changes)

How Has This Been Tested?

• Already covered by automatic testing.
• New test added: (add PR link here).
• Tested by hand on: imxrt1176-nil

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing linter checks and tests passed.
• My changes generate no new compilation warnings for any of the targets.

Special treatment

• This PR needs additional PRs to work (list the PRs, preferably in merge-order).
• I will merge this PR by myself when appropriate.

[github-actions]

Unit Test Results

7 460 tests  ±0   6 745 ✅ ±0   39m 20s ⏱️ +6s
  420 suites ±0     715 💤 ±0 
    1 files   ±0       0 ❌ ±0 

Results for commit 58579b0. ± Comparison against base commit 6f96b6f.