clk: imx: return correct frequency for Ethernet PLL

The i.MX 7 designs Ethernet PLL provides a 1000MHz reference clock.
Store the reference clock in the clk_pllv3 structure according to
the PLL type.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>