MIPS: Better abstract R2300 FPU usage in Kconfig

Introduce a CONFIG_CPU_R2300_FPU Kconfig symbol mirroring the existing
CONFIG_CPU_R4K_FPU, and use it to determine whether to build r4k_fpu.S.

This removes the duplicate R3000 & TX39XX cases in
arch/mips/kernel/Makefile and prepares us for the possibility of
disabling FP support later.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/21004/
Cc: linux-mips@linux-mips.org