i.MX7ULP: Change clock rate calculation for NIC1 BUS and EXT

i.MX7ULP: Change clock rate calculation for NIC1 BUS and EXT

On i.MX7ULP B0, there is change in NIC clock dividers architecture.
On A0, the NIC1 BUS and EXT dividers were in a chain with NIC1 DIV, but
on B0 they are parallel with NIC1 DIV. So now the dividers are independent.
This patch modifies the scg_nic_get_rate function according to this change.

Signed-off-by: Ye Li <ye.li@nxp.com>
Acked-by: Peng Fan <peng.fan@nxp.com>