x86: enable hpet=force for AMD SB400

Add quirk to allow forced usage of HPET on ATI SB400.
I stumbled over machines where HPET is enabled but not reported
by BIOS. This patch configures the HPET base address and makes
it known to the OS.

Signed-off-by: Andreas Herrmann <andreas.herrmann3@amd.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>