soc: xilinx: vcu: add helpers for configuring PLL

The xvcu_set_vcu_pll_info function sets the rate of the PLL and enables
it, which makes it difficult to cleanly convert the driver to the common
clock framework.

Split the function and add separate functions for setting the rate,
enabling the clock and disabling the clock.

Also move the enable of the reference clock from probe to the helper
that enables the PLL.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/20210121071659.1226489-6-m.tretter@pengutronix.de


Signed-off-by: Stephen Boyd <sboyd@kernel.org>