dt-bindings: arm64: bcmbca: Update BCM4908 description

dt-bindings: arm64: bcmbca: Update BCM4908 description

Append "brcm,bcmbca" to BCM4908 chip family compatible strings to
follow the convention of BCMBCA chip and help identifying chip family.

Also add a bare bone generic 4908 board compatbile string to support any
4908 based board. This is useful for board bring-up test and kernel test
with CPU and memory related change.

Signed-off-by: William Zhang <william.zhang@broadcom.com>
Acked-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220803175455.47638-3-william.zhang@broadcom.com
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>