ARM: dts: BCM5301X: Enable UART by default for BCM4708(1), BCM4709(4) & BCM53012

Every device tested so far got UART0 (at 0x18000300) working as serial
console. It's most likely part of reference design and all vendors use
it that way.

It seems to be easier to enable it by default and just disable it if we
ever see a device with different hardware design.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Jon Mason <jon.mason@broadcom.com>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>