MIPS: Clarify the comment for `__cpu_has_fpu'

Reword the comment for `__cpu_has_fpu' to make it unambiguous this code
is for external floating-point units only, generally MIPS I processors
using the original CP1 hardware interface.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/9673/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>