clocksource/drivers/timer-of: Check return value of of_iomap in timer_of_base_init()

clocksource/drivers/timer-of: Check return value of of_iomap in timer_of_base_init()

of_base->base can either be iomapped using of_io_request_and_map() or
of_iomap() depending whether or not an of_base->name has been set.

Thus check of_base->base against NULL as of_iomap() does not return a
PTR_ERR() in case of error.

Fixes: 9aea417afa6b ("clocksource/drivers/timer-of: Don't request the resource by name")
Signed-off-by: Guillaume Ranquet <granquet@baylibre.com>
Link: https://lore.kernel.org/r/20220307172656.4836-1-granquet@baylibre.com
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>