arm: dts: am642-sk: Add initial sk dts

arm: dts: am642-sk: Add initial sk dts

AM642 StarterKit (SK) board is a low cost, small form factor board
designed for TI’s AM642 SoC. It supports the following interfaces:
* 2 GB LPDDR4 RAM
* x2 Gigabit Ethernet interfaces capable of working in switch and MAC mode
* x1 USB 3.0 Type-A port
* x1 UHS-1 capable µSD card slot
* 2.4/5 GHz WLAN + Bluetooth 4.2 through WL1837
* 512 Mbit OSPI flash
* x2 UART through UART-USB bridge
* XDS110 for onboard JTAG debug using USB
* Temperature sensors, user push buttons and LEDs
* 40-pin Raspberry Pi compatible GPIO header
* 24-pin header for peripherals in MCU island (I2C, UART, SPI, IO)
* 54-pin header for Programmable Realtime Unit (PRU) IO pins
* Interface for remote automation. Includes:
	* power measurement and reset control
	* boot mode change

Add basic support for AM642 SK.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>