[MIPS] Fix missing prototypes in asm/fpu.h

While building the Malta defconfig, sparse spat the following
warnings:

>>>>>>>>>>>>>>>>>>
arch/mips/math-emu/kernel_linkage.c:31:6: warning: symbol
'fpu_emulator_init_fpu' was not declared. Should it be static?

arch/mips/math-emu/kernel_linkage.c:54:5: warning: symbol
'fpu_emulator_save_context' was not declared. Should it be
static?

arch/mips/math-emu/kernel_linkage.c:68:5: warning: symbol
'fpu_emulator_restore_context' was not declared. Should it be
static?
>>>>>>>>>>>>>>>>>>

This patch fixes these errors by adding the proper prototypes
to the include/asm-mips/fpu.h header, and actually using this
header in the sparse-spotted source file.

Build-tested with Malta defconfig.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@movial.fi>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>