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/m53evk_defconfig b/configs/m53evk_defconfig
index 1d7933b..a61e2d1 100644
--- a/configs/m53evk_defconfig
+++ b/configs/m53evk_defconfig
@@ -2,3 +2,4 @@
 CONFIG_SYS_EXTRA_OPTIONS="IMX_CONFIG=board/denx/m53evk/imximage.cfg"
 CONFIG_ARM=y
 CONFIG_TARGET_M53EVK=y
+CONFIG_REGEX=y