siemens,am33x,thuban: rename dxr2 to thuban

Update new naming scheme.

Signed-off-by: Samuel Egli <samuel.egli@siemens.com>
Acked-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index 13f579a..0829235 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -346,8 +346,8 @@
 	select CPU_V7
 	select SUPPORT_SPL
 
-config TARGET_DXR2
-	bool "Support dxr2"
+config TARGET_THUBAN
+	bool "Support thuban"
 	select CPU_V7
 	select SUPPORT_SPL