memory: brcmstb: Add driver for DPFE

This driver allows access to DRAM properties, such as the refresh rate,
via the Broadcom STB DDR PHY Front End (DPFE). The refresh rate can be
used as indirect indicator of the DRAM temperature.

The driver also allows setting of the sampling interval.

Signed-off-by: Markus Mayer <mmayer@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>