clk: zynqmp: Add support for get max divider

To achieve best possible rate, maximum limit of divider is required
while computation. Get maximum supported divisor from firmware. To
maintain backward compatibility assign maximum possible value(0xFFFF)
if query for max divisor is not successful.

Signed-off-by: Rajan Vaja <rajan.vaja@xilinx.com>
Link: https://lkml.kernel.org/r/1575527759-26452-5-git-send-email-rajan.vaja@xilinx.com


Acked-by: Michal Simek <michal.simek@xilinx.com>
[sboyd@kernel.org: Remove else return and just return]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>