microblaze: Add comment about reset location

Better to add comment to explain why reset vector points all the time to
origin U-Boot location.
If reset happens U-Boot should start from it's origin location.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/5ca6341b7487708247fe2948d7e496ea6f7c2e02.1655299267.git.michal.simek@amd.com