x86/hpet: Add mode information to struct hpet_channel

The usage of the individual HPET channels is not tracked in a central
place. The information is scattered in different data structures. Also the
HPET reservation in the HPET character device is split out into several
places which makes the code hard to follow.

Assigning a mode to the channel allows to consolidate the reservation code
and paves the way for further simplifications.

As a first step set the mode of the legacy channels when the HPET is in
legacy mode.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Cc: Ashok Raj <ashok.raj@intel.com>
Cc: Andi Kleen <andi.kleen@intel.com>
Cc: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
Cc: Stephane Eranian <eranian@google.com>
Cc: Ravi Shankar <ravi.v.shankar@intel.com>
Link: https://lkml.kernel.org/r/20190623132435.911652981@linutronix.de