cmake: detect RP2350 RISC-V core in rp2 platform automation

[tobiaguiar08]

Summary

Fix the rp2 platform automation to correctly detect and configure RP2350 RISC-V builds instead of silently forcing ARM/CMSIS for all RP2xxx devices.

Problem

The cmake/platform/rp2.cmake module is the platform-automation convenience path for RP2xxx devices. Currently it unconditionally sets CMRX_ARCH arm / CMRX_HAL cmsis and
includes FindCMSIS for every device matching the rp2 prefix, including RP2350 RISC-V. A project using CMRX_DEVICE=rp2350 with PICO_PLATFORM=rp2350-riscv would be silently
misconfigured for the wrong architecture instead of receiving a clear error.

(Note: the documented path and the unmerged integration-test reference project both bypass this automation and set CMRX_ARCH/CMRX_HAL directly, so no real build is currently
broken. This fixes the convenience path for anyone who does use it.)

Changes

• Add an early detection branch keying on PICO_PLATFORM STREQUAL "rp2350-riscv", which sets the correct CMRX_ARCH riscv / CMRX_HAL hal and returns immediately, before any
  ARM/CMSIS logic runs.
• The ARM path below the branch is unchanged; the RP2040 and other ARM RP2xxx variants continue to work exactly as before.

Testing

Verified with a dry cmake configure:

• CMRX_DEVICE=rp2350, PICO_PLATFORM=rp2350-riscv (no PICO_SDK_PATH set) → correctly resolves to CMRX_ARCH=riscv / CMRX_HAL=hal, skips PICO_SDK_PATH check, loads the
  RISC-V HAL module.
• CMRX_DEVICE=rp2040 (no PICO_PLATFORM) → reaches the same PICO_SDK_PATH check and linker-file logic as before, no regression.