ARM: shmobile: rcar-gen2: Make rcar_gen2_dma_contiguous static

make C=1:

    arch/arm/mach-shmobile/setup-rcar-gen2.c:186:12: warning: symbol 'rcar_gen2_dma_contiguous' was not declared. Should it be static?

Make it static, and move it inside the function to avoid a "defined but
not used" warning if CONFIG_DMA_CMA=n.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>