dts: keep clock-names and clocks in SPL DTB if SPL_CLK is enabled

These two properties are necessary for SPL to get clocks from DT.

Note:
For now, only clock look-up by index is supported (clk_get_by_index()
function), so "clock-names" is never parsed in U-Boot.  However, we
may want to support something like clk_get_by_name() in the future,
so let's keep "clock-names" as well as "clocks".

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