Add missing description of 'custom_type_flag' structure member.

Fixes: e605fa9c ("clk: zynqmp: Add support for custom type flags")
Signed-off-by: Rajan Vaja <rajan.vaja@xilinx.com>
Link: https://lore.kernel.org/r/1629720433-19019-1-git-send-email-rajan.vaja@xilinx.comSigned-off-by: Stephen Boyd <sboyd@kernel.org>