gpio/pl061: remove old comment

Since 3ab52475 (gpio: pl061: convert to use generic irq chip) we only have one
spinlock in struct pl061_gpio.

Cc: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>