MIPS: Introduce CONFIG_MIPS_FP_SUPPORT

Introduce a Kconfig variable that will indicate whether to include
support for floating point in the kernel. For now this is always
enabled, and will be made configurable in a later patch.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/21016/
Cc: linux-mips@linux-mips.org