Update Embedded/MCU backend options in README

[digantdesai]

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[Copilot]

Pull request overview

This PR updates the Embedded/MCU backend options in the README's Platform & Hardware Support table to better reflect the current state of supported backends for embedded systems and microcontrollers.

Key Changes:

• Removed XNNPACK from the Embedded/MCU backend list
• Added CMSIS (Cortex-M) backend to the list
• Updated "NXP" to the more specific "NXP Neutron" naming

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[Copilot]

The backend name "CMSIS" may be ambiguous. Throughout the codebase, this backend is consistently referred to as "Cortex-M" (e.g., backends/cortex_m/, EXECUTORCH_BUILD_CORTEX_M, "ARM Cortex-M backend" in documentation). CMSIS-NN is the underlying library used by the Cortex-M backend, not the backend itself. Consider using "Cortex-M" or "ARM Cortex-M" for consistency with the rest of the codebase.

Suggested change

	| Embedded / MCU | ARM Ethos-U, CMSIS, NXP Neutron, Cadence DSP |
	| Embedded / MCU | ARM Ethos-U, Cortex-M, NXP Neutron, Cadence DSP |