ARM: keystone2: keystone_nav: make it dependent on keystone driver

This driver is needed in case if keystone driver is used.
Currently only keystone_net driver uses it. So to avoid
redundant code compilation make the keystone_nav dependent
on keystone net driver. It also leads to compilation errors
for boards that does't use it.

Acked-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>