ARM: u300: retire ancient platforms

This retires the B26/S26, B330/S330 and B365/S365 boards from
the U300 platform. The only board really used anywhere today
is the S335 so let's concentrate any efforts on that one board.
Also the board variants are selected at compile-time, which is
strictly a no-no these days, if multi-board support is to be
brought back it need to happen using run-time and device tree.

My work on ARM Linux started on the B26/S26 so it's a bit sad
to see it go, but there is a time to move on with all things
in life.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>