MIPS: Kconfig: Enable drivers/firmware/Kconfig

This allows the use of /sys/firmware/memmap for MIPS platforms.
kexec-tools may use /sys/firmware/memmap though current versions parse
/proc/iomem.
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/Kconfig

@@ -2585,6 +2585,8 @@ source "net/Kconfig"
 
 source "drivers/Kconfig"
 
+source "drivers/firmware/Kconfig"
+
 source "fs/Kconfig"
 
 source "arch/mips/Kconfig.debug"