spl: imx6: Let spl_boot_device return USDHC1 or USDHC2

Currently, when the spl_boot_device checks the boot device, it
will only return MMC1 when it's either sd or eMMC regardless
of whether or not it's MMC1 or MMC2.  This is a problem when
booting from MMC2 if MMC isn't being manually configured like in
the DM_SPL case with SPL_OF_CONTROL.

This patch will check the register and return either MMC1 or MMC2.

Signed-off-by: Adam Ford <aford173@gmail.com>