platform/x86: force LPS0 functions for AMD

ACPI_LPS0_ENTRY_AMD/ACPI_LPS0_EXIT_AMD are supposedly not
required for AMD platforms, and on some platforms they are
not even listed in the function mask but at least some HP
laptops seem to require it to properly support s0ix.

Based on a patch from Marcin Bachry <hegel666@gmail.com>.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>