arm64: dts: imx8mn: Add spba1 bus

The i.MX8MN has an SPBA bus which covers much of the audio, but
there is a second SPBA bus which covers many of the serial interfaces
like SPI and UARTs currently missing from the device tree. The reference
manual calls the bus handling the audio peripherals SPBA2, and the bus
handling the serial peripherals is called SPBA1.

Rename the existing spba bus to spba2 and add spba1.

Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>