arm64: dts: ti: k3-am64-main: Add main_cpts label

Makes it easier to reference the node in board dts files.

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Link: https://lore.kernel.org/r/20220822095943.18563-1-christian.gmeiner@gmail.com