arm: bcmbca: remove bcm6753 support under CONFIG_ARCH_BCM6753

arm: bcmbca: remove bcm6753 support under CONFIG_ARCH_BCM6753

BCM6753 is essentially same as the main chip BCM6855 but with different
SKU number. Now that BCM6855 is supported under CONFIG_ARCH_BCMBCA and
CONFIG_BCM6855, remove the original ARCH_BCM6753 support and migrate its
configuration and dts settings. This includes:
- Remove the bcm96753ref board folder. It is replaced by the
generic bcmbca board folder.
- Merge the 6753.dtsi setting to the new 6855.dtsi file. Update
96753ref board dts with the new compatible string.
- Delete broadcom_bcm96763ref.h and merge its setting to the new
bcm96855.h file.
- Delete bcm96753ref_ram_defconfig and use a basic config version of
bcm96855_defconfig

Signed-off-by: William Zhang <william.zhang@broadcom.com>
Reviewed-by: Philippe Reynes <philippe.reynes@softathome.com>