ARM: SAMSUNG: make local dev-backlight in mach-s3c64xx

This patch moves dev-backlight from plat-samsung into mach-s3c64xx
because it is used for only s3c64xx no other platforms.

Cc: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>