am335x_boneblack: Only modify NAND/NOR/MMC1 pinmux on BBB in boneblack builds.

In the case of Beaglebone Black we only want to set the NAND or NOR cape
pinmux when the config has been specifically modified by the user for
this non-default case.  Make the default be to set the MMC1 (eMMC)
pinmux.  We don't need similar changes for Beaglebone White as there is
nothing on MMC1 by default there.

Signed-off-by: Tom Rini <trini@ti.com>