mfd: add Atmel Flexcom support ...

This is a wrapper which embeds a SPI controller, a I2C controller and
a USART. Only one function can be used at a time. The choice is done
at boot time by the probe function of this MFD driver according to
a device tree property.

These IP cores are available in the sama5d2 and sam9x60. We already have
support for all three configurations, only thing we need is the MFD
selecting one of them. Add this.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>