arm64: dts: freescale: imx8mm-var-som: Rework WiFi/BT and add legacy dts

arm64: dts: freescale: imx8mm-var-som: Rework WiFi/BT and add legacy dts

The VAR-SOM-MX8MM currently integrates the NXP IW61x wireless module,
providing WiFi over SDIO and Bluetooth over UART.

Move the wireless module configuration out of the base
imx8mm-var-som.dtsi and provide dedicated variant includes.
The IW61x configuration is moved to imx8mm-var-som-wifi-bt-iw61x.dtsi
and used by the Symphony evaluation board device tree.

A separate imx8mm-var-som-wifi-brcm-legacy.dtsi include is added to keep
the configuration for the legacy Broadcom SDIO WiFi module used on
earlier SOM revisions.

To preserve compatibility with older SOM revisions, add a separate
imx8mm-var-som-symphony-legacy.dtb, which disables the IW61x setup and
applies the Broadcom-specific configuration.

The Broadcom-based SOM revision is no longer in production, but support
is kept for existing users.

Signed-off-by: Stefano Radaelli <stefano.r@variscite.com>
Signed-off-by: Frank Li <Frank.Li@nxp.com>