Convert CONFIG_DIMM_SLOTS_PER_CTLR to Kconfig

This converts the following to Kconfig:
   CONFIG_DIMM_SLOTS_PER_CTLR

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/lx2160ardb_tfa_stmm_defconfig b/configs/lx2160ardb_tfa_stmm_defconfig
index c62c873..7f73d80 100644
--- a/configs/lx2160ardb_tfa_stmm_defconfig
+++ b/configs/lx2160ardb_tfa_stmm_defconfig
@@ -62,6 +62,7 @@
 CONFIG_SATA_CEVA=y
 CONFIG_FSL_CAAM=y
 CONFIG_DYNAMIC_DDR_CLK_FREQ=y
+CONFIG_DIMM_SLOTS_PER_CTLR=2
 CONFIG_DDR_ECC=y
 CONFIG_ECC_INIT_VIA_DDRCONTROLLER=y
 CONFIG_MPC8XXX_GPIO=y