ARM: dts: Add support for phyCORE-AM335x SoM

phyCORE-AM335x is a SoM (System on Module) containing
a AM335x SOC. The module can be connected to different
carrier boards.

Some hardware parts are configurable on the phyCORE-AM335x.
So they are disabled on default in this som dtsi file.
They will be enabled in the board dts files, when populated.

* RAM up to 1GiB
* PMIC
* NAND flash up to 1GiB
* Eth PHY on SOM: 1x RMII
* SPI NOR flash 8MiB (optional)
* i2c RTC (optional)
* i2c EEPROM 4kiB (optional)

Signed-off-by: Teresa Remmet <t.remmet@phytec.de>
Signed-off-by: Tony Lindgren <tony@atomide.com>