i2c: Implement i2c_get_chip_by_phandle()

This new function enhances the i2c_get_chip*() toolbox by implementing a
variant that does not require a chip_addr. Instead, the desired device
is pointed to by a phandle.

Signed-off-by: Philip Richard Oberfichtner <pro@denx.de>
Reviewed-by: Heiko Schocher <hs@denx.de>