linker_lists: Add a function to access a linker list entry

linker_lists: Add a function to access a linker list entry

Once declared, you cannot access a linker_list entry since you do not have
a symbol name for it. Add llsym() macro to provide this. This avoids
searching for the symbol at run-time based on name.

An example usage is to declare a driver with U_BOOT_DRIVER(), then obtain
a pointer to that driver later.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>