soc/intel/common/acpi: correct return value for PEPD enum function

soc/intel/common/acpi: correct return value for PEPD enum function

The PEPD enum function returns a bitmask to announce supported/enabled
PEPD functions. Add a comment describing this bitmask and correct the
return value to announce function 1, 5 and 6 as supported.

Also add comments to the disabled functions 3 and 4.

Change-Id: Ib523a54f5ad695e79005aba422282e03f2bc4bed
Signed-off-by: Michael Niewöhner <foss@mniewoehner.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/47140
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>