dm: core: Add a function to read into a unsigned int

dm: core: Add a function to read into a unsigned int

The current dev_read...() functions use s32 and u32 which are convenient
for device tree but not so useful for normal code, which often wants to
use normal integers for values.

Add a helper which supports returning an unsigned int. Also add signed
versions of the unsigned readers.

Signed-off-by: Simon Glass <sjg@chromium.org>