mpc85xx: Add inline GPIO acessor functions

To ease the implementation of other MPC85xx board ports, several common
GPIO helpers are added to <asm/mpc85xx_gpio.h>.

Since each of these compiles to no more than 4-5 instructions it would
be very inefficient to call them out of line, therefore we put them
entirely in the header file.

The HWW-1U-1A board port which these were written for strongly prefers
to set multiple GPIOs as a single batch operation, so the API is
designed around that basis.

To assist other board ports, a small set of wrappers are used which
provides a standard gpio_request() interface around the MPC85xx-specific
functions.  This can be enabled with CONFIG_MPC85XX_GENERIC_GPIO

Signed-off-by: Kyle Moffett <Kyle.D.Moffett@boeing.com>
Cc: Andy Fleming <afleming@gmail.com>
Cc: Peter Tyser <ptyser@xes-inc.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>