x86: i8253: Consolidate definitions of global_clock_event

There are multiple declarations of global_clock_event in header files
specific to particular clock event implementations.  Consolidate them
in <asm/time.h> and make sure all users include that header.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Cc: Venkatesh Pallipadi (Venki) <venki@google.com>
Link: http://lkml.kernel.org/r/20110601180610.762763451@duck.linux-mips.net


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>