mmc: uniphier-sd: skip clock set-up for SPL

mmc: uniphier-sd: skip clock set-up for SPL

The size of SPL is hitting the limit (64KB) for uniphier_v7_defconfig.
When booting from SD/eMMC, obviously its clock has been properly set up
by the boot ROM.  Acutually, no need to re-initialize the clock in SPL.

Using a clock driver would generalize the SoC specific code, but
solving the memory footprint problem would win.

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