doc: memory: fix encodings for spl layout diagrams

doc: memory: fix encodings for spl layout diagrams

The commit 284ef1bbcefc ("doc: memory: Add documentation for system
RAM") added documentation for U-Boot's memory usage along with diagrams
showcasing the SPL's memory usage. Although the SVGs for the diagrams
were improperly encoded.

Therefore, this patch fixes the older SVGs with one's with better
encoding and reduced size created using inkscape[1].

[1]: https://inkscape.org/

Reported-by: Alexander Dahl <ada@thorsis.com>
Fixes: 284ef1bbcefc ("doc: memory: Add documentation for system RAM")
Signed-off-by: Anshul Dalal <anshuld@ti.com>