MLK-17898: mx6sl: Select MX6SL option via Kconfig

Currently it's not possible to build i.MX6SL EVK with CONFIG_SECURE_BOOT
enabled:

In file included from drivers/crypto/fsl/jobdesc.c:12:0:
drivers/crypto/fsl/jobdesc.c: In function ‘inline_cnstr_jobdesc_blob_dek’:
include/fsl_sec.h:268:25: error: ‘CAAM_ARB_BASE_ADDR’ undeclared (first
use in this function)
 #define SEC_MEM_PAGE1  (CAAM_ARB_BASE_ADDR + 0x1000)
                         ^
drivers/crypto/fsl/jobdesc.c:140:21: note: in expansion of macro
‘SEC_MEM_PAGE1’
  memcpy((uint32_t *)SEC_MEM_PAGE1, (uint32_t *)plain_txt, in_sz);
                     ^
include/fsl_sec.h:268:25: note: each undeclared identifier is reported only
once for each function it appears in
 #define SEC_MEM_PAGE1  (CAAM_ARB_BASE_ADDR + 0x1000)
                         ^
drivers/crypto/fsl/jobdesc.c:140:21: note: in expansion of macro
‘SEC_MEM_PAGE1’
  memcpy((uint32_t *)SEC_MEM_PAGE1, (uint32_t *)plain_txt, in_sz);
                     ^
scripts/Makefile.build:280: recipe for target 'drivers/crypto/fsl/jobdesc.o'
failed

Currently the MX6SL option is selected via CONFIG_SYS_EXTRA_OPTIONS, so
CONFIG_FSL_CAAM is being wrongly selected by the imx-common Kconfig.
Select CONFIG_MX6SL via Kconfig to address this issue.

Signed-off-by: Breno Lima <breno.lima@nxp.com>
Reviewed-by: Ye Li <ye.li@nxp.com>