arm64: dts: ti: k3-am64-main: Add ADC nodes

AM64 SoC has a single ADC IP with 8 channels. Add DT node for the same.

Default usecase is to control ADC from non Linux core on the system on
AM642 GP EVM, therefore mark the node as reserved in k3-am642-evm.dts
file. ADC lines are not pinned out on AM642 SK board, therefore disable
the node in k3-am642-sk.dts file.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Link: https://lore.kernel.org/r/20210318113443.20036-1-vigneshr@ti.com