ARM: dts: mmp2: Add Audio SRAM

This memory is region is where the two-channel audio DMA can pump sound
samples into the SSPA's internal FIFO.

Link: https://lore.kernel.org/r/20200718205019.184927-6-lkundrak@v3.sk


Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>