spl: Add a function to determine the U-Boot phase

spl: Add a function to determine the U-Boot phase

U-Boot is built in three phases: TPL, SPL and U-Boot proper. Sometimes
it is necessary to use different init code depending on the phase. For
example, TPL might do very basic CPU init, SPL might do a little more
and U-Boot proper might bring the CPU up to full speed and enable all
cores.

Add a function which allows easy determination of the current phase being
built.

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