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

arch/mips/Kconfig

@@ -2255,9 +2255,13 @@ config CPU_GENERIC_DUMP_TLB
 	bool
 	default y if !(CPU_R3000 || CPU_R8000 || CPU_TX39XX)
 
+config CPU_R2300_FPU
+	bool
+	default y if CPU_R3000 || CPU_TX39XX
+
 config CPU_R4K_FPU
 	bool
-	default y if !(CPU_R3000 || CPU_TX39XX)
+	default y if !CPU_R2300_FPU
 
 config CPU_R4K_CACHE_TLB
 	bool

arch/mips/kernel/Makefile

@@ -42,9 +42,8 @@ sw-$(CONFIG_CPU_TX39XX)		:= r2300_switch.o
 sw-$(CONFIG_CPU_CAVIUM_OCTEON)	:= octeon_switch.o
 obj-y				+= $(sw-y)
 
+obj-$(CONFIG_CPU_R2300_FPU)	+= r2300_fpu.o
 obj-$(CONFIG_CPU_R4K_FPU)	+= r4k_fpu.o
-obj-$(CONFIG_CPU_R3000)		+= r2300_fpu.o
-obj-$(CONFIG_CPU_TX39XX)	+= r2300_fpu.o
 
 obj-$(CONFIG_SMP)		+= smp.o
 obj-$(CONFIG_SMP_UP)		+= smp-up.o