cmd_time: merge run_command_and_time_it with cmd_process

As far as every arch has a get_timer function,
run_command_and_time_it code can now disappear.

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Acked-By: Che-Liang Chiou <clchiou@chromium.org>
[trini: s/ulong/unsigned long/ in command.h portion]
Signed-off-by: Tom Rini <trini@ti.com>