MIPS: Malta: initialise the RTC at boot

The RTC is used on Malta to estimate the clock frequency of the CPU &
optionally the GIC. However the kernel previously did not initialise the
RTC, instead relying upon the bootloader having done so. In order to
minimise dependencies which the kernel has upon the bootloader this
patch causes the kernel to initialise the RTC itself prior to making use
of it.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Reviewed-by: Markos Chandras <markos.chandras@imgtec.com>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6184/