OMAP3: SDRC: Add comments on Errata i520 for Global SW reset

This patch adds comments on precaution to be taken if Global Warm reset is
used as the means to trigger system reset.

Signed-off-by: Vishwanath BS <vishwanath.bs@ti.com>
[paul@pwsan.com: fixed typos, one mentioned by Sanjeev]
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Sanjeev Premi <premi@ti.com>