MIPS: Fix ISA I/II FP signal context offsets

MIPS: Fix ISA I/II FP signal context offsets

Fix a regression introduced with commit 2db9ca0a3551 ("MIPS: Use struct
mips_abi offsets to save FP context") for MIPS I/I FP signal contexts,
by converting save/restore code to the updated internal API.  Start FGR
offsets from 0 rather than SC_FPREGS from $a0 and use $a1 rather than
the offset of SC_FPC_CSR from $a0 for the Floating Point Control/Status
Register (FCSR).

Document the new internal API and adjust assembly code formatting for
consistency.

Signed-off-by: Maciej W. Rozycki <[email protected]>
Cc: Paul Burton <[email protected]>
Cc: [email protected]
Cc: [email protected]
Patchwork: https://patchwork.linux-mips.org/patch/14476/
Signed-off-by: Ralf Baechle <[email protected]>