gpio: mvebu: change compatible string for PWM support

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: Ralph Sennhauser <ralph.sennhauser@gmail.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>