arm64: dts: ti: k3-am64-main: Add ADC DT entry

commit 7ff69a63f3dd11d6656bc436648eaf3edcb15bde from
git://git.ti.com/ti-linux-kernel/ti-linux-kernel.git



AM64x has one ADC instance in main domain.
Add DT entry for the same.

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