asmc: add system state and board identity sysctls

Add dev.asmc.0.system subtree with read-only sysctls for SMC diagnostic
and identity keys: shutdown_cause (MSSD), sleep_cause (MSSP),
thermal_status (MSAL), time_of_day (CLKT), power_state (MSPS),
board_id (RPlt), and chip_gen (RGEN).

Each sysctl is registered only if the key exists on the hardware.

Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D57853