ARM: uniphier: revive accidentally removed dcache_disable()

Commit a8e6300d ("ARM: uniphier: refactor spl_init_board()")
accidentally dropped dcache_disable() call.  Since then, the SPL of
LD11 and LD20 failed to load U-Boot proper.

Fixes: a8e6300d ("ARM: uniphier: refactor spl_init_board()")
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>