ARM: PRIMA2: mv timer to timer-prima2 as we will add timer-marco

Marco timer has different timer IP with prima2, so rename the current timer
to timer-prima2 so that we can add timer-marco.

at the same time, if we don't find prima2 timer node in dt, don't panic the
system as we will make prima2 and marco use same kernel image.

Signed-off-by: Barry Song <Baohua.Song@csr.com>