clk: apn806: Turn the eMMC clock as optional for dts backwards compatible

First version of the binding didn't have the eMMC clock. This patch
allows to not registering the eMMC clock if it is not present in the
device tree. Then the device tree can be backwards compatible.

Suggested-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>