MIPS: ptrace: Make FPU context layout comments match reality

Correct comments across ptrace(2) handlers about an FPU register context
layout discrepancy between MIPS I and later ISAs, which was fixed with
`linux-mips.org' (LMO) commit 42533948caac ("Major pile of FP emulator
changes."), the fix corrected with LMO commit 849fa7a50dff ("R3k FPU
ptrace() handling fixes."), and then broken and fixed over and over
again, until last time fixed with commit 80cbfad7

 ("MIPS: Correct
MIPS I FP context layout").

NB running the GDB test suite for the relevant ABI/ISA and watching out
for regressions is advisable when poking around ptrace(2).

Signed-off-by: Maciej W. Rozycki <macro@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/19326/
Signed-off-by: James Hogan <jhogan@kernel.org>