xtensa: clean up udelay

Replace division with shift, use ccount_freq instead of loops_per_jiffy.
Introduce __MAX_UDELAY and (undefined) __bad_udelay, break build for too
big udelay constants.
Remove irrelevant comment, clean up code style.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
xtensa: implement ndelay

Proper ndelay implementation allows for faster IO rate with drivers that
use ndelay to access their device registers, as otherwise ndelay is
emulated with udelay.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>