powerpc/64: Document the syscall ABI

Add some documentation for the 64-bit syscall ABI, which doesn't seem
to be documented elsewhere.

This attempts to document existing practice. The only small discrepancy
is glibc clobbers not quite matching the kernel (e.g., xer, some
vsyscalls trash cr1 whereas glibc only clobbers cr0). These will be
resolved after this document is merged.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>