clk/zynq/clkc: Add 'fclk-enable' feature

In some use cases Zynq's FPGA clocks are used as static clock
generators for IP in the FPGA part of the SOC for which no Linux driver
exists and would control those clocks. To avoid automatic
gating of these clocks in such cases a new property - fclk-enable - is
added to the clock controller's DT description to accomodate such use
cases. It's value is a bitmask, where a set bit results in enabling
the corresponding FCLK through the clkc.

FPGA clocks are handled following the rules below:

If an FCLK is not enabled by bootloaders, that FCLK will be disabled in
Linux. Drivers can enable and control it through the CCF as usual.

If an FCLK is enabled by bootloaders AND the corresponding bit in the
'fclk-enable' DT property is set, that FCLK will be enabled by the clkc,
resulting in an off by one reference count for that clock. Ensuring it
will always be running.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>