soc: imx: gpcv2: use A_CORE instread of A7 for more i.MX platforms

gpcv2 driver is NOT just used on i.MX7D which has Cortex-A7
cores, but also on i.MX8MQ/i.MX8MM platforms which use Cortex-A53
cores, so let's use A_CORE instread of A7 to avoid confusion.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Acked-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>