ARM: efm32: properly namespace i2c location property

Wolfram Sang pointed out during review of an efm32-i2c driver that the
property to specify the set of pins has a too general name. As several
other efm32 peripherals also have a similar register bit field, add an
"efm32" namespace.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>