arm64: dts: imx95-15x15-evk: Disable PCIe bus in the default dts

arm64: dts: imx95-15x15-evk: Disable PCIe bus in the default dts

Disable the PCIe bus in the default device tree to avoid shared
regulator conflicts between SDIO and PCIe buses. The non-deterministic
probe order between these two buses can break the PCIe initialization
sequence, causing PCIe devices to fail detection intermittently.

On i.MX95-15x15 EVK board, the M.2 connector is physically wired to both
USDHC3 and PCIe0, however the out-of-box module is SDIO IW612 WiFi, so
enable SDIO WiFi in the default imx95-15x15-evk.dts.

Add 'm2_usdhc' label to USDHC3 to support device tree overlay for PCIe
modules. Users who need PCIe can use imx95-15x15-evk-pcie.dtb (added in
a follow-up patch) which applies an overlay to enable PCIe and disable
USDHC3.

Signed-off-by: Sherry Sun <sherry.sun@nxp.com>
Signed-off-by: Frank Li <Frank.Li@nxp.com>