gpio: mvebu: add dbg_show function

This patch adds a dedicated dbg_show function to the gpio-mvebu driver.
In addition to the generic gpiolib informations, this function displays
informations related with the specific Marvell registers (blink enable,
data in polarity, interrupt masks and cause).

Signed-off-by: Simon Guinot <simon.guinot@sequanux.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>