clk: mvebu: ap806: Remove superfluous of_clk_add_provider

While applying the commit a8309ced ("clk: apn806: Add eMMC clock to
system controller driver"), of_clk_add_provider was added wheres it was
already present in the probe function.

This extraneous call is harmless but not useful so remove it.

Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>