powerpc/4xx: Add PowerPC 4xx GPIO driver

This patch adds support for the GPIO functions of PPC40x and PPC44x
SOCs.

Signed-off-by: Steve Falco <sfalco@harris.com>
Acked-by: Stefan Roese <sr@denx.de>
Acked-by: Sean MacLennan <smaclennan@pikatech.com>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>