ARM: dts: at91: sama5d4_ma5d4evk: fix regulator names

vcc_mmc1_reg is a fixed regulators and doesn't need a reg property. Remove
its unit-address.

Solves:
Warning (unit_address_vs_reg): Node /fixedregulator@2 has a unit name, but no reg property

Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>