arm64: dts: ti: Makefile: Use ARCH_K3 for building dtbs

To allow lesser dependency and better maintainability use CONFIG_ARCH_K3
for building dtbs for all K3 based devices. This is as per the
discussion in [0].

[0] https://lore.kernel.org/linux-arm-kernel/20200908112534.t5bgrjf7y3a6l2ss@akan/



Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Suman Anna <s-anna@ti.com>
Link: https://lore.kernel.org/r/20200914162231.2535-2-lokeshvutla@ti.com