Commit dc88403e "iomux-v3: Place pad control definitions into common file" broke
mx51_efikamx because it made i.MX6's pad control definitions conflict with
i.MX51's.

i.MX51's pad control definitions are actually common to some other i.MX
(25/35/53), so move them to the common iomux-v3.h (just like what is done in
Linux's), and select the correct definitions depending on whether CONFIG_MX6 is
defined or not.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>