kconfig: Move REGEX to Kconfig

Having this as a Kconfig allows it to be a dependent feature.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/socfpga_arria5_defconfig b/configs/socfpga_arria5_defconfig
index d658a50..3ef6b88 100644
--- a/configs/socfpga_arria5_defconfig
+++ b/configs/socfpga_arria5_defconfig
@@ -4,3 +4,4 @@
 CONFIG_TARGET_SOCFPGA_ARRIA5=y
 CONFIG_OF_CONTROL=y
 CONFIG_DEFAULT_DEVICE_TREE="socfpga_arria5_socdk"
+CONFIG_REGEX=y