drivers/timer/hpet: Fix default IRQ

The HPET default is to deliver events on the same INTIn as the legacy
PIT IRQ, and in fact our code requires that because it uses the
"legacy routing" option.  So this isn't really a configurable and has
to be set correctly.  Do it right in the kconfig default instead of
forcing boards to set it.

(No, I have no idea where "20" came from either.)

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>