clk: zynqmp: do not export zynqmp_clk_register_* functions

The zynqmp_clk_register_* functions are internal functions of the
driver. Only clkc.c uses these functions to register these clocks.
Therefore, there is no need to export these functions.

The gate and pll already don't export their register_* functions.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>