plat/stm32: Implement fdt_read_uint32_default() as a wrapper

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>
diff --git a/drivers/st/ddr/stm32mp1_ram.c b/drivers/st/ddr/stm32mp1_ram.c
index 273dec0..b21c894 100644
--- a/drivers/st/ddr/stm32mp1_ram.c
+++ b/drivers/st/ddr/stm32mp1_ram.c
@@ -206,13 +206,13 @@
 		return -EINVAL;
 	}
 
-	config.info.speed = fdt_read_uint32_default(node, "st,mem-speed", 0);
-	if (!config.info.speed) {
+	ret = fdt_read_uint32(fdt, node, "st,mem-speed", &config.info.speed);
+	if (ret < 0) {
 		VERBOSE("%s: no st,mem-speed\n", __func__);
 		return -EINVAL;
 	}
-	config.info.size = fdt_read_uint32_default(node, "st,mem-size", 0);
-	if (!config.info.size) {
+	ret = fdt_read_uint32(fdt, node, "st,mem-size", &config.info.size);
+	if (ret < 0) {
 		VERBOSE("%s: no st,mem-size\n", __func__);
 		return -EINVAL;
 	}