boards: nxp: mimxrt1180_evk: Add support for CM7 flash execution

Add support for executing the CM7 core directly from flash memory
(XIP - eXecute In Place) instead of copying to ITCM. This provides
the following benefits:

- Allows for larger code size than the 512KB ITCM limit
- Simplifies memory management for large applications
- Reduces boot time by eliminating the need to copy code to ITCM

The implementation includes:

1. A new Kconfig option CM7_BOOT_FROM_FLASH (default: n) to control
   the execution mode
2. A device tree overlay (cm7_flash_boot.overlay) that configures
   the flash memory for CM7 execution
3. Updates to soc.c to calculate the correct CM7 boot address
   based on the flash partition
5. Documentation updates with instructions for both execution modes

Signed-off-by: Tomas Galbicka <tomas.galbicka@nxp.com>