riscv: dts: microchip: add a device tree for Discovery Kit

riscv: dts: microchip: add a device tree for Discovery Kit

Add a minimal device tree for the Microchip PolarFire SoC Discovery Kit.
The Discovery Kit is a cost-optimized board based on PolarFire SoC
MPFS095T and features:

- 1 GB DDR4x16
- 1x Gigabit Ethernet
- 3x UARTs
- Raspberry Pi connector
- mikroBus connector
- microSD card connector

Link: https://www.microchip.com/en-us/development-tool/mpfs-disco-kit
Signed-off-by: Valentina Fernandez <valentina.fernandezalanis@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>