clocksource/drivers/timer-microchip-pit64b: Add Microchip PIT64B support

Add driver for Microchip PIT64B timer. Timer could be used in continuous
mode or oneshot mode. The hardware has 2x32 bit registers for period
emulating a 64 bit timer. The LSB_PR and MSB_PR registers are used to
set the period value (compare value). TLSB and TMSB keeps the current
value of the counter. After a compare the TLSB and TMSB register resets.
The driver uses PIT64B timer for clocksource or clockevent. First
requested timer would be registered as clockevent, second one would be
registered as clocksource. Individual PIT64B hardware resources were
used for clocksource and clockevent to be able to support high resolution
timers with this hardware implementation.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/1576235962-30123-3-git-send-email-claudiu.beznea@microchip.com