clocksource/drivers/bcm2835_timer: Unmap region obtained by of_iomap

Free memory mapping, if bcm2835_timer_init is not successful.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>