MIPS: Netlogic: Use PIC timer as a clocksource

The XLR/XLS/XLP PIC has a 8 countdown timers which run at the PIC
frequencey. One of these can be used as a clocksource to provide
timestamps that is common across cores. This can be used in place
of the count/compare clocksource which is per-CPU.

On XLR/XLS PIC registers are 32-bit, so we just use the lower 32-bits
of the PIC counter. On XLP, the whole 64-bit can be used.

Provide common macros and functions for PIC timer registers on XLR/XLS
and XLP, and use them to register a PIC clocksource.

Signed-off-by: Jayachandran C <jchandra@broadcom.com>
Patchwork: http://patchwork.linux-mips.org/patch/4786/
Signed-off-by: John Crispin <blogic@openwrt.org>