MIPS: Kconfig: Add Kconfig symbols for EVA support

Add basic Kconfig support for EVA. Not selectable by any platform
at this point.

Signed-off-by: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>
Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>