ARM: u300: convert to multiplatform

Now that we removed our dependency on <mach/*> the U300
can be converted to mutliplatform. Remove the invalid restriction
that U300 would not support AUTO_ZRELADDR (it does) and update
the defconfig in the process.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>