x86/time: calibrate TSC against platform timer

x86/time: calibrate TSC against platform timer

... instead of unconditionally against the PIT. This allows for local
and master system times to remain in better sync (which matters even
when, on any modern system, the master time is really used only during
secondary CPU bringup, as the error between the two is in fact
noticable in cross-CPU NOW() invocation monotonicity).

This involves moving the init_platform_timer() invocation into
early_time_init(), splitting out the few things which really need to be
done in init_xen_time(). That in turn allows dropping the open coded
PIT initialization from init_IRQ() (it was needed for APIC clock
calibration, which runs between early_time_init() and init_xen_time()).

In the course of this re-ordering also set the timer channel 2 gate low
after having finished calibration. This should be benign to overall
system operation, but appears to be the more clean state.

Also do away with open coded 8254 register manipulation from 8259 code.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>