sunxi: sun4i: add missing sdelay() to clock_init_safe()

This delay is required after switching the clock source.

See “A20 Reference manual v1.4” Page 50 / section
“1.5.4.16. CPU/AHB/APB0 CLOCK RATIO”: “If the clock
source is changed, at most to wait for 8 present running
clock cycles.”

This is already implemented in clock_set_pll1(), but was
still missing in clock_init_safe().

Signed-off-by: Ludwig Kormann <ludwig.kormann@ict42.de>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>