arm: mach-k3: Add basic support for AM642 SoC definition

arm: mach-k3: Add basic support for AM642 SoC definition

The AM642 SoC belongs to the K3 Multicore SoC architecture platform,
providing advanced system integration to enable applications such as
Motor Drives, PLC, Remote IO and IoT Gateways.

Some highlights of this SoC are:
* Dual Cortex-A53s in a single cluster, two clusters of dual Cortex-R5F
  MCUs, and a single Cortex-M4F.
* Two Gigabit Industrial Communication Subsystems (ICSSG).
* Integrated Ethernet switch supporting up to a total of two external
  ports.
* PCIe-GEN2x1L, USB3/USB2, 2xCAN-FD, eMMC and SD, UFS, OSPI memory
  controller, QSPI, I2C, eCAP/eQEP, ePWM, ADC, among other
  peripherals.
* Centralized System Controller for Security, Power, and Resource
  Management (DMSC).

See AM64X Technical Reference Manual (SPRUIM2, Nov 2020)
for further details: https://www.ti.com/lit/pdf/spruim2

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>