ARM: dts: at91: vinco: fix regulator name

vcc_3v3_reg is a fixed regulator and doesn't need a reg property. Remove
its unit-address.

Solves:
Warning (unit_address_vs_reg): Node /fixedregulator@0 has a unit name, but no reg property

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>