[ARM] 4408/2: Fixup support for gtwx5715 ixp4xx board

This patch fixes up compiling of the gtwx5715 board setup code,
which has apparently been broken since 2.6.18 and the generic
IRQ changes. In addition it removes some unecessary extern
declarations in the gtwx5715-pci.c file.

Signed-off-by: Michael-Luke Jones <mlj28@cam.ac.uk>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>