release: arm: Enable multicons for arm64

Since we have now EFI framebuffer enabled for ARM64 if we boot on a board
with an screen, u-boot will set up a EFI GOP framebuffer and we won't boot
using the serial console.
Also on RPI3 the firmware always setup the framebuffer area resulting in u-boot
always setup the EFI GOP and FreeBSD never using the serial console.

Reviewed by: gjb, lwshu (previous version)
Differential Revision: https://reviews.freebsd.org/D16472