mips: Add MIPS 74Kc tune

Add MIPS 74Kc tune Kconfig option.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Wills Wang <wills.wang@live.com>
[added missing tune-y entry in arch/mips/Makefile]
Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index 7162f3c..188aaba 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -198,6 +198,9 @@
 config MIPS_TUNE_24KC
 	bool
 
+config MIPS_TUNE_74KC
+	bool
+
 config 32BIT
 	bool