Add pldm bios attrs for huge_pages and power_limit

Make memory_region_size an enum, add huge_page_size and
power_limit_enable and power_limit_in_watts.

Signed-off-by: Corey Swenson <cswenson@us.ibm.com>
Change-Id: Iae47d103df323a30ac8838b00504fac9702feb08