soc: nxp: rt: Add custom memory areas to linker script

RT SOCs have several memory areas (OCRAMs, DTCM, ITCM, SDRAM, FLASH...)
but only two are selected for FLASH (code) and RAM (data).
It would be good to let the linker be aware about other regions, which
could then be used by drivers or application to create dedicated
sections and relocate memory. For example if we have code in ITCM and
data in DTCM, we still need a dma-able region/section for e.g.
video/camera buffers.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>