The FADT signature constant was byte-swapped ("FCAP"), so FADT lookup
always failed. Correct it to "FACP".
SLP_TYP is platform-defined and must come from the DSDT _S5 package.
QEMU/OVMF defines _S5 as 0, so the previous hardcoded value 5 never
triggered S5. Parse _S5 and use its SLP_TYPa/SLP_TYPb for PM1a/PM1b.