sh: Always select RTC_LIB, not just for SUPERH32.

The RTC_LIB helpers are used in arch/sh/kernel/time.c, which was
previously only the case for the 32-bit variant. Now that this has
become the common implementation, move the RTC_LIB select to reflect
that. Fixes up the sh64 build.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>