test: dm: pmbus: add a sandbox PMBus chip emulator

Add a UCLASS_I2C_EMUL driver that emulates a PMBus 1.x compliant chip
behind the sandbox I2C bus, plus the test.dts wiring and sandbox
defconfig that bind it to the generic PMBus regulator
(compatible = "pmbus"). This design is a stub only: it lets the
follow-up dm unit test drive lib/pmbus.c, the generic regulator and
the pmbus CLI command with no real hardware.

The emulator models a flat per-command 16-bit register file and the
three identification block strings (MFR_ID / MFR_MODEL /
MFR_REVISION). READ_IIN and READ_POUT are deliberately left
unimplemented (the chip NAKs them) so the telemetry printer's
"(not supported)" path is exercised, mirroring a real buck that only
calibrates a subset of the sensor classes.

Signed-off-by: Vincent Jardin <[email protected]>
Reviewed-by: Simon Glass <[email protected]>
Signed-off-by: Peng Fan <[email protected]>

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