PH1-LD20 does not support 1.8V signaling for SD card; only Default
Speed and High Speed (up to 50MHz) with 3.3V signaling is supported.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Since no clock driver is implemented for peripherals in U-Boot yet,
this property is needed for the serial driver to set up the divisor
register.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>