boards.h: Remove doc comment reference to nonexistent function

commit b08199c6 accidentally added a reference to a doc
comment to a nonexistent memory_region_allocate_aux_memory().
This was a leftover from a previous version of the patchset
which defined memory_region_allocate_aux_memory() for
"allocate RAM MemoryRegion and register it for migration"
and left "memory_region_init_ram()" with its original semantics
of "allocate RAM MR but do not register for migration". In
the end we decided on the approach of "memory_region_init_ram()
registers the MR for migration, and memory_region_init_ram_nomigrate()
is a new function which does not", but this comment change
got left in by mistake. Revert that part of the commit.

Reported-by: Thomas Huth <huth@tuxfamily.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20180702130605.13611-1-peter.maydell@linaro.org