amdsmu: Expose sysctls for metrics about last sleep

Get "log" address with the SMU_MSG_LOG_GETDRAM_ADDR_HI/LO SMU
commands. From this, we dump & read metrics about residency in various
sleep states (none, S0i2, and S0i3), as well as active durations for
the IP blocks on the CPU during the last sleep. All these metrics are
exposed under the dev.amdsmu.0.metrics node.

This is useful for debugging sleep as the residency registers in the
_LPI object are not populated on some systems (e.g. AMD Phoenix).

A dev.amdsmu.0.ip_blocks.XXX node is also created for each IP block,
with children for the current active state and time spent active during
the last sleep.

Differential Revision: https://reviews.freebsd.org/D48714

Reviewed by: cem, kib, mckusick
Approved by: kib, mckusick
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D48714