acpi/cstate: Allow request deeper C-state, even it has no _CST entry

On laptop, BIOS may unveil more C-states if power cable is unplugged,
which means if you want to use these deeper C-states you will have to
re-adjust C-state sysctls.  This commit automates this kind of
"re-adjust".  Read-only sysctl nodes hw.acpi.cpuX.cx_lowest_use and
hw.acpi.cpu.cx_lowest_use are introduced to show the current deepest
C-state could be used.
acpi/cstate: Make sure we could handle C-state deeper than C3
cpu_idle_hlt: Remove unapplied comment

The comment of possible values is in cpu_idle()
cpuinfo: "Standard Extended Features" -> "Structured Extended Features"
platform/acpi: Update according to ACPI spec 5.0