ARM: mach-shmobile: r8a7740: reserve DMA memory for the frame buffer

The default 2MB size of DMA coherent memory isn't enough for allocate
frame buffer memory.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Tested-by: Simon Horman <horms@verge.net.au>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>