MIPS: Hide CONFIG_MIPS_O32_FP64_SUPPORT prompt for >= MIPSr6

From MIPSr6 onwards FP64 support is mandatory, and so
CONFIG_MIPS_O32_FP64_SUPPORT is always selected for configurations which
support O32 binaries. Hide the useless unchangeable prompt in these
cases.
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/21019/
Cc: linux-mips@linux-mips.org

arch/mips/Kconfig

@@ -2903,7 +2903,7 @@ config SECCOMP
 	  If unsure, say Y. Only embedded should say N here.
 
 config MIPS_O32_FP64_SUPPORT
-	bool "Support for O32 binaries using 64-bit FP"
+	bool "Support for O32 binaries using 64-bit FP" if !CPU_MIPSR6
 	depends on 32BIT || MIPS32_O32
 	help
 	  When this is enabled, the kernel will support use of 64-bit floating