rpi-base: build uart dts overlays by default

[pcurt]

- What I did

I think as we add by default the overlays disable-bt and miniuart-bt to RPI_KERNEL_DEVICETREE_OVERLAYS we also need to add the overlays to enable the uart peripherals. Indeed if we want to use the uart0 peripheral on a Raspberry PI4 with the PL011 with need to enable the following overlays:

dtoverlay=disable-bt
dtoverlay=uart0

- How I did it

Add all the uart dts overlays to RPI_KERNEL_DEVICETREE_OVERLAYS