spl: Add methods to find the position/size of next phase

Binman supports writing the position and size of U-Boot proper and SPL
into the previous phase of U-Boot. This allows the next phase to be easily
located and loaded.

Add functions to return these useful values, along with symbols to allow
TPL to load SPL.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>