soc: xtensa: intel_adsp: fix memory bank shutdown

Amount of memory banks should vary depending on platform, otherwise
power down sequence might result in ipc timeout as the memory used by
the ipc itself is shutdown.

Lift the needed defines from sram.c to adsp_memory.h and shorten the
names to avoid collision with sof code. No functional change to sram.c.

Signed-off-by: Jaska Uimonen <jaska.uimonen@linux.intel.com>