gpio: brcmstb: support gpio-line-names property

gpio: brcmstb: support gpio-line-names property

The default handling of the gpio-line-names property by the
gpiolib-of implementation does not work with the multiple
gpiochip banks per device structure used by the gpio-brcmstb
driver.

This commit adds driver level support for the device tree
property so that GPIO lines can be assigned friendly names.

Signed-off-by: Doug Berger <opendmb@gmail.com>
Link: https://lore.kernel.org/r/1583780521-45702-1-git-send-email-opendmb@gmail.com
Acked-by: Gregory Fong <gregory.0xf0@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>