RealView: Move the timer definitions into the EB specific files

This patch moves the timer definitions from platform.h into board-eb.h
as they are different on PB11MPCore and PB1176. It also adds
timerX_va_base variables in core.c which are set by the
realview_eb_timer_init function before invoking realview_timer_init.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>