board/freescale/common: VSC3316/VSC3308 initialization code

Add code for configuring VSC3316/3308 crosspoint switches
Add README to understand the APIs

   - VSC 3316/3308 is a low-power, low-cost asynchronous crosspoint switch
     capable of data rates upto 11.5Gbps. VSC3316 has 16 input and 16
     output ports whereas VSC3308 has 8 input and 8 output ports.
     Programming of these devices are performed by two-wire or four-wire
     serial interface.

Signed-off-by: Shaveta Leekha <shaveta@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>