The STM32 platform code uses its own set of FDT helper functions,
although some of them are fairly generic.

Remove the implementation of fdt_read_uint32_default() and implement it
on top of the newly introduced fdt_read_uint32() function, then convert
all users over.

This also fixes two callers, which were slightly abusing the "default"
semantic.

Change-Id: I570533362b4846e58dd797a92347de3e0e5abb75
Signed-off-by: Andre Przywara <andre.przywara@arm.com>