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>