ARM: dts: am335x-bone-common: correct mux mode for cmd line

Set pinmux_emmc_pins mux mode for cmd line to MODE2 in order
to detect eMMC on BBB and BBW + eMMC cape.

Signed-off-by: Balaji T K <balajitk@ti.com>
Tested-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Benoit Cousson <bcousson@baylibre.com>