ARM: dts: stm32: make the LTDC clock usable by the clock driver

As described in [1], the "clocks" property contains "a phandle to the
clock device node, an index selecting between gated clocks (0) and other
clocks (1), and an index specifying the clock to use." The current version
of the clock driver, unlike the kernel, is currently able to properly
handle nodes with "clocks" properties with an index set to 0.

This patch is preparatory for future developments that require the use
of the LTDC clock.

[1] Documentation/devicetree/bindings/clock/st,stm32-rcc.txt
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>