Add code to calculate the CPU and PIC frequency for XLP2XX SoCs.

Since the PIC frequency on XLP2XX can be configured, add a new macro
pic_timer_freq() to be used in netlogic/common/time.c.
Signed-off-by: Jayachandran C <jchandra@broadcom.com>
Cc: linux-mips@linux-mips.org
Cc: Ganesan Ramalingam <ganesanr@broadcom.com>
Patchwork: http://patchwork.linux-mips.org/patch/5701/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>