arm64: dts: imx8mp: Switch WiFI enable signal to mmc-pwrseq-simple on i.MX8MP DHCOM SoM

The reset-gpio is connected to WL_REG_EN signal of the WiFi MAC, the
mmc-pwrseq-simple driver is better suited to operate this signal as
it is tied to the slot instead of the MAC, and it can enable the MAC
before the brcmfmac driver binds to it. Make use of the MMC power
sequencer.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>