kernel: Add a "nocache" read-write memory section

Add a "nocache" read-write memory section that is configured to
not be cached. This memory section can be used to perform DMA
transfers when cache coherence issues are not optimal or can not
be solved using cache maintenance operations.

This is currently only supported on ARM Cortex M7 with MPU.

Fixes #2927

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>