HACK: mmc: dw_mmc-exynos: implementing hwreset on shutdown for eMMC

Use GPIO to issue a hwreset of the eMMC on system shutdown/reboot, so
that the eMMC stays useable once the systems comes up again.
Needed for the Hardkernel ODROID-X2/U2/U3.
HACK: ARM: dts: exynos4412-odroid: add eMMC reset GPIO spec

Add the hwreset GPIO property, so that the Exynos4412 based
ODROID-X2/U2/U3 boards properly detect the eMMC after a reboot.