fix(stm32mp15-fdts): use /omit-if-no-ref/ for spi and i2c

fix(stm32mp15-fdts): use /omit-if-no-ref/ for spi and i2c

Use /omit-if-no-ref/ keyword in DT to remove extra device nodes only
when they are not used / not referenced.

If the board device tree only defines subnodes, dtc does not consider it
as usage, you have to specifically mention device's phandle, e.g.:

\ {
	i2c6-phandle = <&i2c6>;
};

or in aliases section
aliases {
	i2c6 = &i2c6;
};

Signed-off-by: Vyacheslav Yurkov <uvv.mail@gmail.com>
Change-Id: I431ecd93576f97fd021d82d23b93c659fc8f26b8