arch-mx6: fix MX6_PAD_DECLARE macro to work with MX6 duallite

if we build for an i.mx6 (d)ual(l)ite CONFIC_MX6DL we shall use
MX6DL_PAD instead the common MX6_PAD.

Signed-off-by: Hannes Schmelzer <oe5hpm@oevsv.at>