i2c: add fwnode APIs

Add fwnode APIs for finding and getting I2C adapters, which will be
used by the SFP code. These are passed the fwnode corresponding to
the adapter, and return the I2C adapter. It is the responsibility of
the caller to find the appropriate fwnode.

We keep the DT and ACPI interfaces, but where appropriate, recode them
to use the fwnode interfaces internally.

Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Wolfram Sang <wsa@kernel.org>