Provide a new function mips_gictimer_get_freq() which returns the
frequency at which a GIC timer will count. This will be useful for
boards which perform setup based upon this frequency.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Reviewed-by: Leon Alrae <leon.alrae@imgtec.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Yongbok Kim <yongbok.kim@imgtec.com>