am335x/ti814x: Correct MMC_BOOT_DEVICES_START/END

Given that on TI814x we have MMC1/2 swapped, we also need to swap them
in MMC_BOOT_DEVICES_START/END

Reported-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Tom Rini <trini@ti.com>