arm64: dts: Move BCM4908 dts to bcmbca folder

As part of ARCH_BCM4908 to ARCH_BCMBCA migration, move the BCM4908 dts
files to bcmbca folder and use CONFIG_ARCH_BCMBCA to build all the
BCM4908 board dts. Delete bcm4908 folder and its makefile as well.

Signed-off-by: William Zhang <william.zhang@broadcom.com>
Link: https://lore.kernel.org/r/20220803175455.47638-5-william.zhang@broadcom.com


Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>