dm: core: Inline a few ofnode functions in SPL

A recent change to unify the flattree/livetree code introduced a small
size increase in SPL on some boards. For example SPL code size for
px30-core-ctouch2-px30 increased by 40 bytes.

To address this we can take advantage of the fact that some of the ofnode
functions are only called a few times in SPL, so it is worth inlining
them.

Add new Kconfig options to control this. These functions are not inlined
for U-Boot proper, since this increases code size.

Fixes: 2ebea5ea ("dm: core: Combine the flattree and livetree binding code")
Signed-off-by: Simon Glass <sjg@chromium.org>