dts: socfpga: Update clock entry to support multiple parents

The periph_pll and sdram_pll can have multiple parents. Update the device tree
to list all the possible parents for the PLLs. Add an entry for the the
f2s_sdram_ref_clk, which is a possible parent for the sdram_pll.

Also remove the clock-frequency entry in the f2s_periph_ref_clk, as this
property should be placed in dts file.

Signed-off-by: Dinh Nguyen <dinguyen@altera.com>
Cc: Mike Turquette <mturquette@linaro.org>
Cc: Steffen Trumtrar <s.trumtrar@pengutronix.de>