ARM: multi_v7_defconfig: Enable serial console on RPi 3

The Raspberry Pi 3 uses the 8250-based auxilary UART of the BCM2837 SoC
as the primary serial console of the board, so enable it in the
multiplatform defconfig.

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Eric Anholt <eric@anholt.net>