The 7278 device is the first device that includes support for the V7
memory map developed for use in 64-bit architecture brcmstb devices.
This map relocates the register physical offset from 0xF0000000 to
0x0000000008000000.

Since the ARM PERIPHBASE value is also relocated in the V7 memory map
we can use its value to determine whether this device uses the new
V7 memory map and therefore where to look for the SUN_TOP_CTRL
register used to identify the chip family.
Signed-off-by: Doug Berger <opendmb@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>