MIPS: Move 'gic_frequency' to common location.

Move the global variable 'gic_frequency' to be defined in the file
'arch/mips/kernel/irq-gic.c' instead of defining it individually
for each platform making use of the GIC. Also change the type to
be an unsigned integer instead of signed.

Signed-off-by: Steven J. Hill <Steven.Hill@imgtec.com>