gpio: mvebu: only fail on missing clk if pwm is actually to be used

The gpio IP on Armada 370 at offset 0x18180 has neither a clk nor pwm
registers. So there is no need for a clk as the pwm isn't used anyhow.
So only check for the clk in the presence of the pwm registers. This fixes
a failure to probe the gpio driver for the above mentioned gpio device.

Fixes: 757642f9

 ("gpio: mvebu: Add limited PWM support")
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>