board/orangepi: add support for orangepi-pc-plus board

This board is a variant of orangepi-pc board. It adds 8GB eMMC and Realtek
RTL8189FTV SDIO WiFi chip. This variant has a separate support in U-Boot
and kernel. So it makes sense to create a separate entry
for this board in buildroot.

Board support package includes the following components:
 - mainline u-boot 2017.09
 - mainline kernel 4.13.11

Signed-off-by: Sergey Matyukevich <geomatsi@gmail.com>
[Thomas: add entry to DEVELOPERS file.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
8 files changed