asmc: add MMIO backend for T2 Macs

[guest-seuros]

Summary

T2 Macs (2018+) expose the SMC via memory-mapped registers instead of
I/O ports. Add asmcmmio.c/asmcmmio.h implementing the MMIO transport:
key read/write, getinfo, getbyindex, and a poll-based wait with
exponential backoff.

The driver probes for MMIO at attach time by checking the LDKN firmware
version key; if MMIO is available it is used, otherwise the standard
I/O port backend is used.

T2 fan speeds use IEEE 754 floats instead of fpe2 fixed-point.
Per-fan manual mode uses F%dMd keys instead of the FS! bitmask.
Battery charge limit is exposed via dev.asmc.N.battery_charge_limit.

Tested on:

MacBookPro16,2 (A2251, iBridge2,10)
MacBookPro15,4 (A2159, iBridge2,8)
MacBookAir8,2  (A1932, iBridge2,5)
Mac mini 8,1   (A1993, iBridge2,7)
iMac20,2       (A2115, iBridge2,16)
iMacPro1,1     (A1862, iBridge1,1)