ARM: u300: merge u300.c into core.c and rid headers

This gets rid of the separate u300.c file in mach-u300 since
it can just as well live right in core.c, then we also get
rid of the broadcasted <mach/platform.h> file that is not
helping anyone. Put the interface to the system timer into
a separate header.

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