ARM: dts: Add minimal Raspberry Pi 4 support

This adds minimal support for the new Raspberry Pi 4 without the
fancy stuff like GENET, PCIe, xHCI, 40 bit DMA and V3D. The RPi 4 is
available in 3 different variants (1, 2 and 4 GB RAM), so leave the memory
size to zero and let the bootloader take care of it. The DWC2 is still
usable as peripheral via the USB-C port.

Other differences to the Raspberry Pi 3:
- additional GIC 400 Interrupt controller
- new thermal IP and HWRNG
- additional MMC interface (emmc2)
- additional UART, I2C, SPI and PWM interfaces
- clock stretching bug in I2C IP has been fixed

Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Acked-by: Eric Anholt <eric@anholt.net>
Acked-by: Florian Fanelli <f.fainelli@gmail.com>