dt-bindings: i2c: i2c-imx-lpi2c: add ipg clk

i.MX LPI2C actually requires dual clock: per clock and ipg clock, so add
both.

Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>