ARM: shmobile: Add shared R-Car Gen2 CMA reservation code

Add R-Car Gen2 CMA memory reservation code that can be
shared between multiple SoCs and boards. At this point
r8a7790 and r8a7791 are supported.

The top 256MiB of the legacy 32-bit physical memory space
is assigned to a separate CMA area that may be assigned
to various devices later on.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
[horms+renesas@verge.net.au: rebased]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>