dm: core: Add functions to read 8/16-bit integers

Add functions to read 8/16-bit integers like the existing functions for
32/64-bit to simplify read of 8/16-bit integers from device tree
properties.

Signed-off-by: Stefan Herbrechtsmeier <stefan.herbrechtsmeier@weidmueller.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>