[ARM] nommu: fixups for the exception vectors

The high page vector (0xFFFF0000) does not supported in nommu mode.
This patch allows the vectors to be 0x00000000 or the begining of DRAM
in nommu mode.

Signed-off-by: Hyok S. Choi <hyok.choi@samsung.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>