MLK-13360 mx6sll_arm2: Fix typo issue in mx6sll LPDDR2 ARM2 defconfig

Change the 'kONFIG' to 'CONFIG', otherwise will get build warning: unexpected data

Signed-off-by: Ye Li <ye.li@nxp.com>
(cherry picked from commit 4deeb824)

configs/mx6sll_lpddr2_arm2_defconfig

-kONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/freescale/mx6sll_arm2/imximage_lpddr2.cfg,LPDDR2"
+CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/freescale/mx6sll_arm2/imximage_lpddr2.cfg,LPDDR2"
 CONFIG_ARM=y
 CONFIG_ARCH_MX6=y
 CONFIG_TARGET_MX6SLL_ARM2=y