Add support for BeagleBoard.org PocketBeagle

Add support for BeagleBoard.org PocketBeagle

Texas Instruments AM3358 based low-cost board using Octavo Systems OSD3358 SIP
with built-in TPS65217 PMIC and 512MB DDR3. Board features small 35mm x
55mm size, high-speed USB OTG, microSD and 72 0.1" expansion header
pins with 2xSPI, 2xI2C, 2xUART, USB, 8xADC, up-to-44 GPIO, PRU pins and much more.

https://beagleboard.org/pocket

This was tested using the am335x_evm_usbspl_defconfig.

Note that MII pins are enabled despite not having Ethernet on this
board. This avoids an issue where otherwise many timeout errors would be
generated. See https://e2e.ti.com/support/arm/sitara_arm/f/791/t/298976
for some related discussion.

Signed-off-by: Jason Kridner <jdk@ti.com>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Tom Rini <trini@konsulko.com>