clocksource: Mmio: remove artificial 32bit limitation

The EP93xx is registering a clocksource of 40 bits with
clocksource_mmio_init() but this is not working because of this
artificial limitation. It works fine to lift the uppe limit to
64 bits, and since cycle_t is u64, it should intuitively have been
like that from the beginning.

Fixes: 000bc178

 "ARM: ep93xx: switch to GENERIC_CLOCKEVENTS"
Reported-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: http://lkml.kernel.org/r/1449768101-6879-1-git-send-email-linus.walleij@linaro.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>