drivers: misc: Fixes: Rename CONFIG_SPL_SOCFPGA_SEC_REG to CONFIG_SPL_SOCFPGA_DT_REG

Commit 3f190c55
("drivers: misc: Add socfpga_dtreg driver for Intel SoCFPGA")

This commit rename CONFIG_SPL_SOCFPGA_SEC_REG to CONFIG_SPL_SOCFPGA_DT_REG
in Makefile.

Signed-off-by: Wan Yee Lau <wan.yee.lau@intel.com>