arm: arch_timer: remove redundant available check

This check is a holdover from the pre-devicetree days. As the timer
is not probed except by platforms which register it via devicetree,
it's not strictly necessary.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>