configs: freescale_imx31_3stack: fix ARM variant selection

Commit 24dfbe71e (arch/arm: do not distinguish revisions of ARM1136JF-S)
removed the r0 variant of the arm1136jf-s, but didn't update the defconfig -
So it ends up using arm926 instead.

Fix it by selecting the correct symbol instead.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

configs/freescale_imx31_3stack_defconfig

@@ -1,6 +1,6 @@
 # architecture
 BR2_arm=y
-BR2_arm1136jf_s_r0=y
+BR2_arm1136jf_s=y
 BR2_ARM_EABIHF=y
 
 # Linux headers same as kernel, a 3.15 series