rename symbol: CONFIG_STM32 -> CONFIG_ARCH_STM32

Have this symbol follow the pattern of all other such symbols.

Signed-off-by: Trevor Woerner <twoerner@gmail.com>
diff --git a/configs/stm32f746-disco_defconfig b/configs/stm32f746-disco_defconfig
index cdfa8bb..be030bf 100644
--- a/configs/stm32f746-disco_defconfig
+++ b/configs/stm32f746-disco_defconfig
@@ -1,5 +1,5 @@
 CONFIG_ARM=y
-CONFIG_STM32=y
+CONFIG_ARCH_STM32=y
 CONFIG_SYS_TEXT_BASE=0x08008000
 CONFIG_SYS_MALLOC_F_LEN=0xE00
 CONFIG_ENV_SIZE=0x2000