drivers: mbox: make mbox_nxp_imx_mu use PRE_KERNEL_1 init level

The MU init should be very early for some platforms. For examples,
i.MX95 is using MU for ARM SCMI. It should be initialzied early
for next power, pinctrl, clock etc. management.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>