ARM: timer-sp: allow getting timer1 clock from DT to fallback to legacy clock

The sp804 clocks may be specified in DT, but the kernel may still be using
legacy clocks. This is handled if a single clock for sp804 is present,
but not when 3 clocks are present.

This prevents Versatile platforms from breaking when the DT has clocks
added.

Signed-off-by: Rob Herring <robh@kernel.org>
Cc: Russell King <linux@arm.linux.org.uk>