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/Kconfig b/arch/arm/Kconfig
index bd802b3..5864646 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -576,7 +576,7 @@
 	select CPU_ARM926EJS
 	select SUPPORT_SPL
 
-config ORION5X
+config ARCH_ORION5X
 	bool "Marvell Orion"
 	select CPU_ARM926EJS