ARM: dts: at91: sama5d4_xplained: fix regulator names

vcc_3v3_reg and vcc_mmc1_reg are fixed regulators and don't need a reg
property. Remove their unit-address.

Solves:
Warning (unit_address_vs_reg): Node /fixedregulator@0 has a unit name, but no reg property
Warning (unit_address_vs_reg): Node /fixedregulator@1 has a unit name, but no reg property

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