ARM: uniphier: deassert RST_n of eMMC device for LD11/LD20

For LD11 and LD20 SoCs, the RST_n pin is asserted by default.  If
the EXT_CSD[162], bit[1:0] (RST_n_ENABLE) is fused, the eMMC device
would stay in the reset state until its RST_n pin is deasserted by
software.

Currently, this is cared by an ad-hoc way because the eMMC hardware
reset provider is not supported in U-Boot for now.  This code should
be re-written once the "mmc-pwrseq-emmc" binding is supported.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>