pine64-pinebookpro: Use linux-firmware packages

The firmware-pine64-pinebookpro predates the firmware for this device being available upstream. It is now packaged in two separate packages: linux-firmware-brcm and linux-firmware-rockchip.

Install those two linux-firmware packages and drop the custom one. This drop a custom firmware package and also ensure that firmware is kept up to date with the rest of linux-firmware-*.

The resulting installation includes all the previously included firmware files and I've confirmed that both sound and wifi work fine (dmesg indicates bluetooth is available too).

cc: @craftyguy