stm32: move stm32 specific code to mach-stm32

This patch moves stm32 sources at one place, with this armv7m now contains
only generic stuff.

Signed-off-by: Vikas Manocha <vikas.manocha@st.com>
diff --git a/arch/arm/Makefile b/arch/arm/Makefile
index 3d15673..cd7d880 100644
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -56,6 +56,7 @@
 machine-$(CONFIG_ARCH_S5PC1XX)		+= s5pc1xx
 machine-$(CONFIG_ARCH_SOCFPGA)		+= socfpga
 machine-$(CONFIG_ARCH_ROCKCHIP)		+= rockchip
+machine-$(CONFIG_STM32)			+= stm32
 machine-$(CONFIG_TEGRA)			+= tegra
 machine-$(CONFIG_ARCH_UNIPHIER)		+= uniphier
 machine-$(CONFIG_ARCH_ZYNQ)		+= zynq