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Summary

Add the BOOM workload-side pieces for the NutShell-style emu/FPGA split:

• boom-mem2g: emu profile, SimAXIUart at 0x10000000;
• boom-fpga-mem2g: FPGA profile, board UART16550 at 0x310b0000,
  reg-shift=2, clock-frequency=50MHz;
• gcc16 SPEC2006 config for rv64gc_zba_zbb_zbs.

Commits

• feat(dts): add the boom-mem2g board profile
• feat(spec2006): add a gcc16 cfg for rv64gc_zba_zbb_zbs
• feat(dts): add the BOOM FPGA UART16550 profile

Validation

• Both DTS profiles generate and compile.
• linux/hello DEFAULT_DTB=boom-fpga-mem2g builds successfully with
  serial0: serial@310b0000, reg-shift=2, and
  earlycon=uart8250,mmio,0x310b0000.
• The emu boom-mem2g profile keeps the existing 0x10000000 UART.

Notes

• The FPGA board wrapper must expose BOOM MMIO AXI to S00_AXI; the board
  already has axi_uart16550 at 0x310b0000.

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see #83