ARM: dts: imx7s: Correct GPT's ipg clock source

i.MX7S/D's GPT ipg clock should be from GPT clock root and
controlled by CCM's GPT CCGR, using correct clock source for
GPT ipg clock instead of IMX7D_CLK_DUMMY.

Fixes: 3ef79ca6

 ("ARM: dts: imx7d: use imx7s.dtsi as base device tree")
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>