clk: zynq: pll: Fix kernel-doc warnings

Fix the following kernel-doc warning

drivers/clk/zynq/pll.c:15: warning: missing initial short description on line:
 * struct zynq_pll
drivers/clk/zynq/pll.c:96: warning: No description found for return value of 'zynq_pll_is_enabled'
drivers/clk/zynq/pll.c:116: warning: No description found for return value of 'zynq_pll_enable'
drivers/clk/zynq/pll.c:187: warning: No description found for return value of 'clk_register_zynq_pll'

Signed-off-by: Shubhrajyoti Datta <shubhrajyoti.datta@xilinx.com>
Link: https://lore.kernel.org/r/9929a56462bfdd491c43c233abc4341fc14dac1d.1637139796.git.shubhrajyoti.datta@xilinx.com


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