rename symbol: CONFIG_ORION5X -> CONFIG_ARCH_ORION5X

Have this symbol follow the pattern of all other such symbols.
This patch removes a TODO from the code.

Signed-off-by: Trevor Woerner <twoerner@gmail.com>
diff --git a/arch/arm/mach-orion5x/Kconfig b/arch/arm/mach-orion5x/Kconfig
index 7644b8d..5baa6fb 100644
--- a/arch/arm/mach-orion5x/Kconfig
+++ b/arch/arm/mach-orion5x/Kconfig
@@ -1,4 +1,4 @@
-if ORION5X
+if ARCH_ORION5X
 
 choice
 	prompt "Marvell Orion board select"