imx6: Re-add mmc aliases

In commit d0399a46e7cda63c07e3eb8558bef84cfb068028, the device tree was
synchronized from linux and the aliases were dropped.

They need to be kept so that the mmc cards are in the right order.
Without the aliases, u-boot reports:
MMC:   FSL_SDHC: 2, FSL_SDHC: 3

With the aliases, u-boot reports:
MMC:   FSL_SDHC: 0, FSL_SDHC: 1

The upstream linux device tree does not contain the same aliases than
u-boot (It keeps the devices order with /dev/mmcblk2 and /dev/mmcblk3).
Because this board has been using different aliases in u-boot
and linux, a imx6q-sabrelite-u-boot.dtsi file is added to be
automatically included in imx6q-sabrelite.dts.

This way, linux and u-boot each keep their own aliases and there
is no breakage and current installations.

This should never be done for new boards as we want to keep linux and
u-boot with the same aliases as much as possible.
This patch is only necessary to avoid breaking existing setups.

Signed-off-by: Detlev Casanova <detlev.casanova@collabora.com>