board: rockchip: add Radxa ROCK5B Rk3588 board

ROCK 5B is a Rockchip RK3588 based SBC (Single Board Computer) by Radxa.

There are tree variants depending on the DRAM size : 4G, 8G and 16G.

Specification:

Rockchip Rk3588 SoC
4x ARM Cortex-A76, 4x ARM Cortex-A55
4/8/16GB memory LPDDR4x
Mali G610MC4 GPU
MIPI CSI 2 multiple lanes connector
eMMC module connector
uSD slot (up to 128GB)
2x USB 2.0, 2x USB 3.0
2x HDMI output, 1x HDMI input
Ethernet port
40-pin IO header including UART, SPI, I2C and 5V DC power in
USB PD over USB Type-C
Size: 85mm x 54mm

Kernel commits:
a1d3281450ab ("arm64: dts: rockchip: Add rock-5b board")
6fb13f888f2a ("arm64: dts: rockchip: Update sdhci alias for rock-5b")

Signed-off-by: Eugen Hristev <eugen.hristev@collabora.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>