ARM: dts: pxa: add pwri2c to pxa device-tree

pxa27x variant has 2 I2C busses on the SoC :
 - the casual I2C
 - the power I2C, normally driving power regulators, and capable of
 receiving orders on core frequency modifications

Add the missing pwri2c to pxa27x description.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>