ARM: u300: hide submenus

Right now the U300 submenus are showcased for everyone even if
we're not on v5 multiplatforms. Hide this in the multiplatform
configuration properly.

Cc: arm@kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>