arm: lpc32xx: add CONFIG_ARCH_LPC32XX build option

The explicit arch specific build symbol allows to group supported
boards, generalize common config options and it will serve as
a dependency for platform only drivers.

Two related board defconfigs are resynced after the change.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
diff --git a/arch/arm/cpu/arm926ejs/lpc32xx/Kconfig b/arch/arm/cpu/arm926ejs/lpc32xx/Kconfig
new file mode 100644
index 0000000..986ad73
--- /dev/null
+++ b/arch/arm/cpu/arm926ejs/lpc32xx/Kconfig
@@ -0,0 +1,20 @@
+if ARCH_LPC32XX
+
+config SYS_SOC
+	default "lpc32xx"
+
+choice
+	prompt "NXP LPC32xx board select"
+
+config TARGET_DEVKIT3250
+	bool "Timll DevKit3250"
+
+config TARGET_WORK_92105
+	bool "Work Microwave Work_92105"
+
+endchoice
+
+source "board/timll/devkit3250/Kconfig"
+source "board/work-microwave/work_92105/Kconfig"
+
+endif