ARM: 6619/1: nommu: avoid mapping vectors page when !CONFIG_MMU

ARM: 6619/1: nommu: avoid mapping vectors page when !CONFIG_MMU

When running without an MMU, we do not need to install a mapping for the
vectors page. Attempting to do so causes a compile-time error because
install_special_mapping is not defined.

This patch adds compile-time guards to the vector mapping functions
so that we can build nommu configurations once more.

Acked-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>