arm64: dts: ti: k3-am642-evm: Enable ADC

commit bee3b2a7efb325e0fb0e8596a491b054584f516f from
git://git.ti.com/ti-linux-kernel/ti-linux-kernel.git



AM64x EVM has 8 ADC input channels available on a header.
Therefore add DT entries to enable the same.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Xulin Sun <xulin.sun@windriver.com>