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.