plat/stm32: Use generic fdt_read_uint32_array() implementation

plat/stm32: Use generic fdt_read_uint32_array() implementation

The device tree parsing code for the STM32 platform is using its own FDT
helper functions, some of them being rather generic.
In particular the existing fdt_read_uint32_array() implementation is now
almost identical to the new generic code in fdt_wrappers.c, so we can
remove the ST specific version and adjust the existing callers.

Compared to the original ST implementation the new version takes a
pointer to the DTB as the first argument, and also swaps the order of
the number of cells and the pointer.

Change-Id: Id06b0f1ba4db1ad1f733be40e82c34f46638551a
Signed-off-by: Andre Przywara <andre.przywara@arm.com>