corstone700: splitting the platform support into FVP and FPGA ...

This patch performs the following:

- Creating two corstone700 platforms under corstone700 board:

  fvp and fpga

- Since the FVP and FPGA have IP differences, this commit provides a specific DTS for each platform
- The platform can be specified using the TARGET_PLATFORM Makefile variable
(possible values are: fvp or fpga)
- Allowing to use u-boot by:
  - Enabling NEED_BL33 option
  - Fixing non-secure image base: For no preloaded bl33 we want to
    have the NS base set on shared ram. Setup a memory map region
    for NS in shared map and set the bl33 address in the area.
- Setting the SYS_COUNTER_FREQ_IN_TICKS based on the selected
platform
- Setting ARM_MAP_SHARED_RAM and ARM_MAP_NS_SHARED_RAM to use MT_MEMORY

Change-Id: I4c8ac3387acb1693ab617bcccab00d80e340c163
Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Abdellatif El Khlifi <abdellatif.elkhlifi@arm.com>