ARM: dts: imx6*-hummingboard2: split out eMMC support

eMMC is optional on HB2 boards, and may be implemented by the SOM.  Move
it out of the base HB2 include file.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>