ARM: imx: limit errata selection to Cortex-A9 based designs

The two erratas 754322 and 775420 are Cortex-A9 specific. The i.MX 6UL
SoCs include a Cortex-A7 CPU and hence do not need this erratas enabeld.
This patch moves the errata selection from the family Kconfig symbol to
the SoC specifc Kconfig symbols where a Cortex-A9 is used.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>