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/ls1028aqds_tfa_lpuart_defconfig b/configs/ls1028aqds_tfa_lpuart_defconfig
index 73caf98..e692004 100644
--- a/configs/ls1028aqds_tfa_lpuart_defconfig
+++ b/configs/ls1028aqds_tfa_lpuart_defconfig
@@ -54,6 +54,7 @@
 CONFIG_SATA_CEVA=y
 CONFIG_FSL_CAAM=y
 # CONFIG_DDR_SPD is not set
+CONFIG_DIMM_SLOTS_PER_CTLR=2
 CONFIG_MPC8XXX_GPIO=y
 CONFIG_DM_I2C=y
 CONFIG_I2C_SET_DEFAULT_BUS_NUM=y