soc: avtimer: add support for decimal value MHz clock

Add support for clocks which has a decimal MHz frequency (e.g. 19.2MHz).
The support is added in such a way that this will avoid the need of
having a floating point operation to get the msec count from the
number of clock ticks.

Change-Id: Iee57a5b67139150a1ffb86e580d8a90e032a894f
Signed-off-by: Banajit Goswami <bgoswami@codeaurora.org>
Signed-off-by: Meng Wang <mwang@codeaurora.org>