clk: ux500: clk-sysctrl: handle clocks with no parents

Fix clk_reg_sysctrl() to set main clock registers of new struct
clk_sysctrl even if the registered clock has no parents.

This fixes an issue where "ulpclk" was registered with all clk->reg_*
fields uninitialized, causing a -EINVAL error from clk_prepare().

Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Acked-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Mike Turquette <mturquette@linaro.org>