device-pine64-pinephone: fix emmc booting
This fixes the new booting in u-boot when the boot device is eMMC, the MMC numbers in u-boot just happened to match up for the SD card in linux (mmc0 in u-boot == mmcblk0 in linux)
The eMMC storage doesn't match up since u-boot skips "unused" busses so mmc1 in u-boot is mmcblk2 in linux since mmc1 is used by wifi in linux.