Allow GPIO_BASIC_MMIO_CORE to be used to provide an accessor library
for implementing GPIO drivers whilst abstracting the register access
detail.  Based on a patch from Anton Vorontsov[1] and adapted to allow
bgpio_chip to be embedded in another structure.

Changes since v1:
	- Register the gpio_chip in the platform device probe

1. https://lkml.org/lkml/2011/4/19/401Signed-off-by: NAnton Vorontsov <cbouatmailru@gmail.com>
Signed-off-by: NJamie Iles <jamie@jamieiles.com>
Signed-off-by: NGrant Likely <grant.likely@secretlab.ca>