arm64: dts: imx8mp: Bind bluetooth UART on DH electronics i.MX8M Plus DHCOM

The i.MX8MP DHCOM SoM does contain muRata 2AE WiFi+BT chip, bind the
bluetooth to UART2 using btbcm and hci_bcm drivers. Use PLL3 to drive
UART2 clock divided down to 64 MHz to obtain suitable block clock for
exact 4 Mbdps, which is the maximum supported baud rate by the muRata
2AE BT UART.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>