linker: arm: Add cortex_m itcm section

Adds a linker section for Cortex-M instruction tightly coupled memory
(ITCM), similar to the existing section for DTCM. A new executable MPU
region is not added as there isn't currently a need to make this section
accessible to user mode. This section can be enabled by setting a device
tree chosen node zephyr,itcm.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>