ARM64: dts: vexpress: Use assigned-clock-parents for sp810

The sp810 clk driver is calling the clk consumer APIs from
clk_prepare ops to change the parent to a 1 MHz fixed rate clock
for each of the clocks that the driver provides. Use
assigned-clock-parents for this instead of doing it in the driver
to avoid using the consumer API in provider code. This also
allows us to remove the usage of clk provider APIs that take a
struct clk as an argument from the sp810 driver.

Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Olof Johansson <olof@lixom.net>