arm: mvebu: Update CRS305-1G-4S board flash layout

Update the MikroTik CRS305-1G-4S flash layout to support redundant UBI
partitions.
Additionally enable the UBI commands in crs305-1g-4s_defconfig.

Signed-off-by: Luka Kovacic <luka.kovacic@sartura.hr>
Cc: Luka Perkov <luka.perkov@sartura.hr>
Cc: Jakov Petrina <jakov.petrina@sartura.hr>
Reviewed-by: Stefan Roese <sr@denx.de>