Add a way of checking the position of a structure member

Add a way of checking the position of a structure member

U-Boot uses structures for hardware access so it is important that these
structures are correct. Add a way of asserting that a structure member is
at a particular offset. This can be created using the datasheet for the
hardware.

This implementation uses Static_assert() since BUILD_BUG_ON() only works
within functions.

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