asmc: add raw SMC key read/write interfaceHEADmain

This patch adds a debugging interface to read and write arbitrary
Apple SMC keys by name through sysctl, enabling hardware exploration
 and control of undocumented features.

The interface provides four sysctls under dev.asmc.0.raw.*:
  - key - Set the 4-character SMC key name (e.g., "AUPO")
  - value - Read/write key value as a hex string
  - len - Auto-detected key value length (can be overridden)
  - type - Read-only 4-character type string (e.g., "ui8", "flt")

Implementation includes a new asmc_key_getinfo() function using SMC
command 0x13 to query key metadata. The interface automatically
detects key lengths and types, uses hex string encoding for
arbitrary binary values, and is safe for concurrent access via
CTLFLAG_NEEDGIANT.

This interface was essential for discovering that the AUPO key
enables Wake-on-LAN from S5 state, and for mapping all 297 SMC keys
on Mac Mini 5,1.

Reviewed by:	ngie, adrian, markj
Differential Revision:	https://reviews.freebsd.org/D54441

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