i2c: ihs: Get rid of fpgamap

Since the IHS I2C driver want upstream, the surrounding infrastructure
has changed quite a bit (notably, the fpgamap driver was replaced with a
regmap driver).

Update the driver to work with these changes.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
Reviewed-by: Heiko Schocher <hs@denx.de>