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: NFabio Estevam <fabio.estevam@nxp.com>

arch/arm/mach-imx/mx5/Kconfig

@@ -8,9 +8,11 @@ config MX5
 config MX51
 	bool
 	select SYS_FSL_ERRATUM_ESDHC_A001
+	select ARM_CORTEX_A8_CVE_2017_5715
 
 config MX53
 	bool
+	select ARM_CORTEX_A8_CVE_2017_5715
 
 choice
 	prompt "MX5 board select"