Convert CONFIG_DAVINCI_SPI to Kconfig

This converts the following to Kconfig:
   CONFIG_DAVINCI_SPI

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/configs/da850_am18xxevm_defconfig b/configs/da850_am18xxevm_defconfig
index 824c383..0675401 100644
--- a/configs/da850_am18xxevm_defconfig
+++ b/configs/da850_am18xxevm_defconfig
@@ -42,4 +42,5 @@
 CONFIG_DM_SERIAL=y
 CONFIG_SYS_NS16550=y
 CONFIG_DM_SPI=y
+CONFIG_DAVINCI_SPI=y
 # CONFIG_FAT_WRITE is not set