doc: board: fix Microchip MPFS Icicle Kit doc

Two sibling headings (here eMMC) cannot have the same title.

Warning, treated as error:
doc/board/microchip/mpfs_icicle.rst:423:duplicate label
board/microchip/mpfs_icicle:emmc, other instance in
doc/board/microchip/mpfs_icicle.rst
make[1]: *** [doc/Makefile:69: htmldocs] Error 2

* Correct the heading levels.
* Add missing empty lines after headings.

Fixes: 9e550e18 ("doc: board: Add Microchip MPFS Icicle Kit doc")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>