dm: add dev_read_u32()

dm: add dev_read_u32()

dev_read_u32_default() always returns something even when the property
is missing.  So, it is impossible to do nothing in the case.  One
solution is to use ofnode_read_u32() instead, but adding dev_read_u32()
will be helpful.

BTW, Linux has an equvalent function, device_property_read_u32();
it is clearer that it reads a property.  I cannot understand the
behavior of dev_read_u32() from its name.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>