net: ethernet: ti: cpts: switch to readl/writel_relaxed()

Switch to readl/writel_relaxed() APIs, because this is recommended
API and the CPTS IP is reused on Keystone 2 SoCs
where LE/BE modes are supported.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>