arm: mvebu: turris_omnia: Specify VHV gpio for eFUSE programming

VHV gpio is connected to MCU and only on updated board design. Without it
eFUSE programming does not work. Omnia MCU driver exports this GPIO to
U-Boot under name mcu_56 and only when it is supported by MCU. So U-Boot
fuse command refuse eFUSE programming on older board design when VHV gpio
is not available.

We tested that Armada 385 without connected VHV gpio can do eFUSE
programming but only for some bits and only sometimes - it is unstable.
And better to be disabled on older board design without VHV gpio support.

Signed-off-by: Pali Rohár <pali@kernel.org>