arm: bcmbca: Move BCM63138 ARCH_BCM_63XX to ARCH_BCMBCA

Remove ARCH_BCM_63XX Kconfig for BCM63138 and merge its selections to
ARCH_BCMBCA. Delete bcm63xx.c as it is no longer needed.

Signed-off-by: William Zhang <william.zhang@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>