powerpc: Rename plapr routines to plpar

Back in 2013 we added some hypercall wrappers which misspelled
"plpar" (P-series Logical PARtition) as "plapr".

Visually they're hard to distinguish and it almost doesn't matter, but
it is confusing when grepping to miss some calls because of the typo.

They've also started spreading, so before they take over let's fix
them all to be "plpar".

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>