i2c: at91: segregate master mode specific code from probe and init func

In order to implement slave mode support for the at91 hardware we have to
segregate all master mode specific function parts from the general parts.
The upcoming slave mode patch will call its sepcific probe resp. init
function instead of the master mode functions after the shared general
code has been executed.

This concept has been influenced by the i2c-designware driver.

Signed-off-by: Juergen Fitschen <me@jue.yt>
Signed-off-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>