As it turns out more than just Armada 370 and XP support using GPIO
lines as PWM lines. For example the Armada 38x family has the same
hardware support. As such "marvell,armada-370-xp-gpio" for the
compatible string is a misnomer.

Change the compatible string to "marvell,armada-370-gpio" before the
driver makes it out of the -rc stage. This also follows the practice of
using only the first device family supported as part of the name.

Also update the documentation and comments in the code accordingly.

Fixes: 757642f9 ("gpio: mvebu: Add limited PWM support")
Signed-off-by: NRalph Sennhauser <ralph.sennhauser@gmail.com>
Acked-by: NGregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>