serial/bcm63xx_uart: use correct alias naming

The bcm63xx_uart driver uses the of alias for determing its id. Recent
changes in dts files changed the expected 'uartX' to the recommended
'serialX', breaking serial output. Fix this by checking for a 'serialX'
alias as well.

Fixes: e3b992d0 ("MIPS: BMIPS: Improve BCM6328 device tree")
Fixes: 2d52ee82 ("MIPS: BMIPS: Improve BCM6368 device tree")
Fixes: 7537d273 ("MIPS: BMIPS: Add device tree example for BCM6358")
Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>