ARM: dts: enable WLAN for Mecer Xtreme Mini S6

ARM: dts: enable WLAN for Mecer Xtreme Mini S6

The Mecer Xtreme Mini S6 features a wireless module, based on a
Realtek 8723BS, which provides WLAN and Bluetooth connectivity via
SDIO and UART interfaces respectively.

Define a simple MMC power sequence that declares the GPIO pins
connected to the module's WLAN Disable and Bluetooth Disable pins
as active low reset signals, because both signals must be deasserted
for WLAN radio operation.

Configure the host's SDIO interface for High Speed mode with 1.8v
I/O signalling and IRQ detection over a 4-bit wide bus.

Signed-off-by: Justin Swartz <justin.swartz@risingedge.co.za>
Link: https://lore.kernel.org/r/20200406135006.23759-1-justin.swartz@risingedge.co.za
Signed-off-by: Heiko Stuebner <heiko@sntech.de>