Add Kbuild code to allow the builder to select which GPTIMER block to use
for the Linux clockevents timer.  Practical choices at this point are
GPTIMER1 or GPTIMER12.  Both of these timers are in the WKUP powerdomain,
and so are unaffected by chip power management.  GPTIMER1 can use sys_clk
as a source, for applications where a high-resolution timer is more important
than power management.  GPTIMER12 has the special property that it has
the secure 32kHz oscillator as its source clock, which may be less prone
to glitches than the off-chip 32kHz oscillator.  But on HS devices, it may
not be available for Linux use.

At this point, it appears that most boards are fine with GPTIMER1, but
Beagle should use GPTIMER12.
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>