Initial xG24 support

[mananni-silabs]

Preliminary support for xG24 Silicon Labs platform.

• Radio model is functional for 802.15.4-based stacks (Zigbee, Open Thread, Connect and other RAIL-based proprietary solutions), however on certain platforms we are currently experiencing thread safety issues resulting in PC corruption. Also, because of a dependency on the CortexM class, it can not currently be compiled using the existing flow of including the file in the Peripherals.csproj file. We instead pull the file in from .resc file directly.
• SEMAILBOX only implements commands needed for 802.15.4 stacks and does not actually perform any encryption for now
• FlashUserData only implements EUI64 manufacturer token for now
• MPU use currently results into the application not initializing correctly, so apps should be built without MPU support

[CLAassistant]

All committers have signed the CLA.

[mars-low]

It may be necessary to explicitly initialize Vector Table Offset, because otherwise the communication breaks after pause/start. During resuming after pause, InitPCAndSP() is called in OnResume() and VectorTableOffset for sequencer should be manually initialized prior to this to not be overridden by the automatically deduced address of the first non null section.

Suggested change

	sequencer.SP = machine.SystemBus.ReadDoubleWord(SequencerMemoryBaseAddress);
	sequencer.VectorTableOffset = SequencerMemoryBaseAddress;
	sequencer.SP = machine.SystemBus.ReadDoubleWord(SequencerMemoryBaseAddress);

[mananni-silabs]

@mars-low thank you for the suggestion. I will give this a try