gplugd: Adding mmc support for gplugd

a) Define MFPs for MMC1 and MMC2.
b) Enable EXT4 support.
c) Adding new mmc driver pxa_sdh, used from
   non-upstream uboot source. Can be found
   at http://www.plugcomputer.org/downloads/gplugd/
   see. uboot_gplugd_v0.6_final.tar.gz

Signed-off-by: Yogi P <yogi@vadactro.org.in>