VECTORS_BASE displays the exception base address. Now on no-MMU as
the exception base address is dynamically estimated, define
VECTORS_BASE to the variable holding it.

As it is the case, limit VECTORS_BASE constant definition to MMU.
Suggested-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: afzal mohammed <afzal.mohd.ma@gmail.com>
Tested-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>