clocksource/drivers/orion: Add delay_timer implementation

Add an implementation for the ARM delay timer, which is used for
udelay().  This provides less CPU dependent and more accurate delays -
the CPU loop on Marvell Dove appears to calibrate to around 6% too
short.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>