bcm53xx: rework patch enabling UART support

Follow upstream patch and handle it using &uart0. Also disable &uart1 as
it's most likely unused. This will allow us to get valuable reports and
upstream these changes.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46140 3c298f89-4303-0410-b956-a3cf2f4a3e73