ARM: dts: exynos: Fix MMC regulator on Arndale5250 board

According to the schematic, both eMMC and SDMMC use dedicated fixed
regulators connected directly to the DC5V and MAIN_DC rails. Remove the
GPX1-1 line assigned to the MMC regulator, because such control
connection doesn't exist. Also change its name to VDD_MMC to avoid
conflict with LDO18 output of S5M8767 PMIC.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>