SolidRun/Cn913xCEx7Eval: Add platform support

This patch adds the required platform description files, along with
the hardware configuration libraries, for the SolidRun
CN913x CEx7 Evaluation Board. Supported interfaces:

* SPI flash & memory-mapped variable storage access
* uSD
* eMMC
* 7x PCIE root complex
* USB
* Networking:
  * 1Gbps RGMII via PHY
  * 2500Base-X via quad 1Gpbs switch
  * 5Gbps via SFP cage and PHY

Signed-off-by: Marcin Wojtas <mw@semihalf.com>