[MIPS] Atlas: Fix building the RTC driver

Atlas maps its RTC chip in the host mmio space rather than using the
"traditional" location in the PCI/ISA port space.  A change that has
happened to the generic RTC header requires to define ARCH_RTC_LOCATION
now.

Signed-off-by: Maciej W. Rozycki <macro@mips.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>