[MIPS] unexport rtc_mips_set_time()

No users for the rtc_mips_set_time() routine exist outside of the
core kernel code. Therefore, EXPORT_SYMBOL(rtc_mips_set_time) is
useless, and this patch removes it.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>