OMAP2/3: SRAM: add comment about crashes during a TLB miss

Some users were observing crashes during the execution of CORE DVFS
code from OCM RAM -- a locally-modified copy of the linux-omap code.
Richard Woodruff tracked this down to a DTLB miss which had been
inadvertently and intermittently caused by the local modifications.
(The TLB miss caused the ARM MMU to attempt to walk the page tables
stored in SDRAM, which was not possible since SDRAM is off-line for a
portion of the CORE DVFS OCM RAM code.)

Add a note to the OMAP2 & OMAP3 CORE DVFS SRAM code to warn others that
changes may result in crashes here if they are not carefully tested.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Richard Woodruff <r-woodruff2@ti.com>
Cc: Jon Hunter <jon-hunter@ti.com>
Cc: Nishanth Menon <nm@ti.com>