dm: Fix error handling for dev_read_addr_ptr

dev_read_addr_ptr had different semantics depending on whether OF_LIVE was
enabled. This patch converts both implementations to return NULL on error,
and converts all call sites which check for FDT_ADDR_T_NONE to check for
NULL instead. This patch also removes the call to map_physmem, since we
have dev_remap_addr* for those semantics.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/core/read.c b/drivers/core/read.c
index 85b3b7a..8bb456b 100644
--- a/drivers/core/read.c
+++ b/drivers/core/read.c
@@ -167,7 +167,7 @@
 {
 	fdt_addr_t addr = dev_read_addr(dev);
 
-	return (addr == FDT_ADDR_T_NONE) ? NULL : map_sysmem(addr, 0);
+	return (addr == FDT_ADDR_T_NONE) ? NULL : (void *)(uintptr_t)addr;
 }
 
 void *dev_remap_addr(const struct udevice *dev)