arm: bcmbca: remove bcm68360 support under CONFIG_ARCH_BCM68360

arm: bcmbca: remove bcm68360 support under CONFIG_ARCH_BCM68360

BCM68360 is a variant within the BCM6856 chip family. Now that BCM6856
is supported under CONFIG_ARCH_BCMBCA and CONFIG_BCM6856, remove the
original ARCH_BCM68360 support and migrate its configuration and dts
settings. This includes:
  - Remove the bcm968360bg board folder. It is replaced by the generic
    bcmbca board folder.
  - Merge the 68360.dtsi setting to the new 6856.dtsi file. Update board
    dts with the new compatible string.
  - Merge broadcom_bcm968360bg.h setting to the new bcm96856.h file.
  - Remove bcm968360bg_ram_defconfig as a basic config version of
    bcm96856_defconfig is now added.

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