arm, arm926ejs: Add option CONFIG_SYS_EXCEPTION_VECTORS_HIGH

The V bit of the c1 register of CP15 should not be cleared on DA850
SoCs since they have no valid memory at 0x00000000. This patch
introduces a configuration option CONFIG_SYS_EXCEPTION_VECTORS_HIGH
that allows setting the correct value for the V bit.

Signed-off-by: Christian Riesch <christian.riesch@omicron.at>
Reported-by: Sughosh Ganu <urwithsughosh@gmail.com>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Tom Rini <trini@ti.com>
Cc: Sughosh Ganu <urwithsughosh@gmail.com>
Cc: Heiko Schocher <hs@denx.de>