CLOCKSOURCE: Add Pistachio clocksource-only driver

CLOCKSOURCE: Add Pistachio clocksource-only driver

The Pistachio SoC provides four general purpose timers, and allow
to implement a clocksource driver.

This driver can be used as a replacement for the MIPS GIC and MIPS R4K
clocksources and sched clocks, which are clocked from the CPU clock.

Given the general purpose timers are clocked from an independent clock,
this new clocksource driver will be useful to introduce CPUFreq support
for Pistachio machines.

Signed-off-by: Ezequiel Garcia <[email protected]>
Signed-off-by: Govindraj Raja <[email protected]>
Reviewed-by: Thomas Gleixner <[email protected]>
Acked-by: Daniel Lezcano <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: Andrew Bresticker <[email protected]>
Cc: James Hartley <[email protected]>
Cc: Damien Horsley <[email protected]>
Cc: James Hogan <[email protected]>
Cc: Ezequiel Garcia <[email protected]>
Cc: Ezequiel Garcia <[email protected]>
Patchwork: https://patchwork.linux-mips.org/patch/10899/
Signed-off-by: Ralf Baechle <[email protected]>