clk: uniphier: fix unmet direct dependencies warning

Since commit f0776a55 ("spl: dm: Kconfig: SPL_CLK depends on
SPL_DM"), the following warning is displayed:

  $ make uniphier_v8_defconfig
  warning: (ARCH_ZYNQ && ARCH_ZYNQMP && STM32F7 && CLK_UNIPHIER) selects
  SPL_CLK which has unmet direct dependencies (CLK && SPL_DM)

While I am here, I am removing the prompt to make it user-unconfigurable
option so that "select CLK_UNIPHIER" can be omitted.

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