boards: lpcxpresso55s36: fix declared memory region sizes

- Fixes declared memory region sizes in .yaml and .rst.
- Adds SRAMX linker region.
- The lpcxpresso55s36 linker memory regions are:
  FLASH =246KB, RAM=112KB, SRAMX=16KB.

Signed-off-by: Andrej Butok <andrey.butok@nxp.com>