dt-bindings: riscv: spacemit: add MusePi Pro board

dt-bindings: riscv: spacemit: add MusePi Pro board

Document the compatible string for the MusePi Pro [1]. It is a 1.8-inch
single board computer based on the SpacemiT K1/M1 RISC-V SoC [2].

Here's a refined list of its core features:
  - SoC: SpacemiT M1/K1, 8-core 64-bit RISC-V.
  - Memory: LPDDR4X @ 2400MT/s, available in 8GB & 16GB options.
  - Storage: Onboard eMMC 5.1 (64GB/128GB options), M.2 M-Key for NVMe
             SSD (2230 size), and a microSD slot (UHS-II) for expansion.
  - Display: HDMI 1.4 (1080P@60Hz) and 2-lane MIPI DSI FPC (1080P@60Hz).
  - Connectivity: Onboard Wi-Fi 6 & Bluetooth 5.2, single Gigabit Ethernet
                  port (RJ45).
  - USB: 4x USB 3.0 Type-A (host) and 1x USB 2.0 Type-C (device/OTG).
  - Expansion: Full-size miniPCIe slot and a second M.2 M-Key (2230).
  - GPIO: Standard 40-pin GPIO interface.
  - MIPI: 1x 4-lane MIPI CSI FPC and 2x MIPI DSI FPC interfaces.
  - Clock: Onboard RTC with battery support.

Link: https://developer.spacemit.com/documentation?token=YJtdwnvvViPVcmkoPDpcvwfVnrh&type=pdf
Link: https://www.spacemit.com/en/key-stone-k1
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Troy Mitchell <troy.mitchell@linux.spacemit.com>
Link: https://lore.kernel.org/r/20251023-k1-musepi-pro-dts-v4-1-01836303e10f@linux.spacemit.com
Signed-off-by: Yixun Lan <dlan@gentoo.org>