drivers: edma: allow transfer descriptors to be placed in SRAM

SOCs using the EDMA IP that supported caching must locate EDMA transfer
control descriptors (TCDs) in non cacheable memory. For M7 cores, this
can simply use the "nocache" section. For M4 cores, where the nocache
section does not exist, the chosen SRAM section must be a tightly
coupled memory block which cannot be cached. Add a note to all boards
with M4 SOCs that support caching explaining this issue, and enable EDMA
driver to locate TCDs in SRAM.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>