Convert CONFIG_DA8XX_GPIO to Kconfig

This converts the following to Kconfig:
   CONFIG_DA8XX_GPIO

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/configs/da850_am18xxevm_defconfig b/configs/da850_am18xxevm_defconfig
index 9283bf9..abb2295 100644
--- a/configs/da850_am18xxevm_defconfig
+++ b/configs/da850_am18xxevm_defconfig
@@ -35,6 +35,7 @@
 CONFIG_OF_CONTROL=y
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_DM=y
+CONFIG_DA8XX_GPIO=y
 CONFIG_DM_I2C=y
 CONFIG_DM_I2C_COMPAT=y
 CONFIG_MTD_DEVICE=y