spl: Add support for a relocating jump to the next phase

When one xPL phase wants to jump to the next, the next phase must be
loaded into its required address. This means that the TEXT_BASE for the
two phases must be different and there cannot be any memory overlap
between the code used by the two phases. It also can mean that phases
need to be moved around to accommodate any size growth.

Having two xPL phases in SRAM at the same time can be tricky if SRAM
is limited, which it often is. It would be better if the second phase
could be loaded somewhere else, then decompressed into place over the
top of the first phase.

Introduce a relocating jump for xPL to support this. This selects a
suitable place to load the (typically compressed) next phase, copies
some decompression code out of the first phase, then jumps to this code
to decompress and start the next phase.

This feature makes it much easier to support Verified Boot for Embedded
(VBE) on RK3399 boards, which have 192KB of SRAM.

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

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