clk: atlas7: fix pll missed divide NR in fraction mode

PLL VCO frequency is given by Fvco = Fref * 2 * NF / NR
in integer-N mode, or by Fvco = Fref * SSN / NR in Spread
Spectrum (fractional-N) mode. Thus fix the missing part
of NR

Signed-off-by: Guo Zeng <guo.zeng@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>