ARM: 6000/1: removing compilation warning comming from <asm/irq.h>

irq.h is using struct pt_regs *. Due to this compilation
warning is comming. Removing this warning by adding declaration
of struct pt_regs.

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>