gpu: ipu-v3: Kconfig: Remove SOC_IMX6SL from IMX_IPUV3_CORE Kconfig

SOC_IMX6SL does not have the IPU block, so remove it from the Kconfig entry.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>