board: Add uCRobotics Bubblegum-96 board support

This commit adds uCRobotics Bubblegum-96 board support. This board is
one of the 96Boards Consumer Edition platform based on Actions Semi
S900 SoC.

Features:
- Actions Semi S900 SoC (4xCortex A53, Power VR G6230 GPU)
- 2GiB RAM
- 8GiB eMMC, uSD slot
- WiFi, Bluetooth and GPS module
- 2x Host, 1x Device USB port
- HDMI
- 20-pin low speed and 40-pin high speed expanders, 6 LED, 3 buttons

U-Boot will be loaded by ATF at EL2 execution level. Relevant driver
support will be added in further commits.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>