ARM: shmobile: r8a7794: Reserve memory as other R-Car Gen2 SoCs

Other R-Car Gen2 SoCs such as r8a7790 and r8a7791 reserve
the top 256 MiB of memory for use with CMA. Adjust the
board-less r8a7794 code to do the same.

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