libpayload: Generalize and redistribute timekeeping code

libpayload: Generalize and redistribute timekeeping code

The timekeeping code in libpayload was dependent on rdtsc, and when it was
split up by arch, that code was duplicated even though it was mostly the same.
This change factors out actually reading the count from the timer and the
speed of the timer and puts the definitions of ndelay, udelay, mdelay and
delay into generic code. Then, in x86, the timer_hz and timer_get_raw_value
functions which used to be in depthcharge were moved over to libpayload's
arch/x86/timer.c. In ARM where there isn't a single, canonical timer, those
functions are omitted with the intention that they'll be implemented by a
specific timer driver chosen elsewhere.

Change-Id: I9c919bed712ace941f417c1d58679d667b2d8269
Signed-off-by: Gabe Black <gabeblack@google.com>
Reviewed-on: http://review.coreboot.org/2717
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>