dm: core: use u32 instead of int for dev_read_u32_default

dm: core: use u32 instead of int for dev_read_u32_default

dev_read_u32_default is for getting a u32 from a Device Tree property
and allows to take a default value if that property is missing.

Considering it calls ofnode_read_u32_default which takes a u32 and
returns a u32, it should do the same instead of using an int, especially
considering that int size is typically architecture-specific, as opposed
to u32.

This incidentally matches all other dev_read_*_default functions (except
dev_read_s32_default which will be tackled in the next commit).

Fixes: 47a0fd3bad38 ("dm: core: Implement live tree 'read' functions")
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Simon Glass <sjg@chromium.org>