arch: bcm281xx: Initial commit of bcm281xx architecture code

Add bcm281xx architecture support code including a clock framework and
chip reset.  Define register block base addresses for the bcm281xx
architecture and create an empty gpio header file required when
CONFIG_CMD_GPIO is set.

Signed-off-by: Darwin Rambo <drambo@broadcom.com>
Reviewed-by: Steve Rae <srae@broadcom.com>
Reviewed-by: Tim Kryger <tkryger@linaro.org>