[ARM] 4820/1: RealView: Select the timer IRQ at run-time

This patch sets the timer IRQ at run-time by moving the sys_timer
structure and the timer_init function to the realview_eb.c file. This
allows multiple RealView platforms to be compiled in the same kernel
image.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>