arm64: dts: broadcom: bcm4908: Fix UART clock name

According to the binding the correct clock name is "refclk".

Fixes: 2961f69f ("arm64: dts: broadcom: add BCM4908 and Asus GT-AC5300 early DTS files")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>