riscv: dts: microchip: add pic64gx and its curiosity kit

riscv: dts: microchip: add pic64gx and its curiosity kit

The Curiosity-GX10000 (PIC64GX SoC Curiosity Kit) is a compact SoC
prototyping board featuring a Microchip PIC64GX SoC PIC64GC-1000.
Features include:
- 1 GB DDR4 SDRAM
- Gigabit Ethernet
- microSD-card slot

note: due to issue on some board, the SDHCI is limited to HS (High
speed mode, with a clock of 50MHz and 3.3V signals).

Signed-off-by: Pierre-Henry Moussay <pierre-henry.moussay@microchip.com>
Co-developed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>