ARM: davinci: da8xx: Add CFGCHIP syscon platform device

The CFGCHIP registers are used by a number of devices, so use a syscon
device to share them. The first consumer of this will be the phy-da8xx-usb
driver.

Add the syscon device and register it.

Signed-off-by: David Lechner <david@lechnology.com>
[nsekhar@ti.com: minor commit message fixes]
Signed-off-by: Sekhar Nori <nsekhar@ti.com>