ARM: ep93xx: switch to GENERIC_CLOCKEVENTS

This switches the EP93xx to use GENERIC_CLOCKEVENTS and
CLKSRC_MMIO. Also implements a sched_clock() hook.
Tested on the SIM.ONE. Use only oneshot events.

Tested-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>