gpio: generalize for all generic gpio providers

The Blackfin gpio command isn't terribly Blackfin-specific.  So generalize
the few pieces into two new optional helpers:
	name_to_gpio() - turn a string name into a GPIO #
	gpio_status() - display current pin bindings (think /proc/gpio)

Once these pieces are pulled out, we can relocate the cmd_gpio.c into the
common directory.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>