mx5: Select ARM_CORTEX_A8_CVE_2017_5715

On a 4.18-rc1 kernel the following warning is seen on i.MX51 and
i.MX53:

CPU0: Spectre v2: firmware did not set auxiliary control register IBE bit, system vulnerable

Select the ARM_CORTEX_A8_CVE_2017_5715 workaround for i.MX51/i.MX53
to fix the problem.

With this patch applied the kernel reports:

CPU0: Spectre v2: using BPIALL workaround

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>