drivers: clk: zynqmp: Allow zero divisor value

Zero divider is valid and default for some of ZynqMP
clocks. Allow zero divisor when CLK_DIVIDER_ALLOW_ZERO
for the clock is set.

Signed-off-by: Rajan Vaja <rajanv@xilinx.com>
Signed-off-by: Jolly Shah <jollys@xilinx.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>