amba: Add Kconfig file

Rather than duplicate the ARM_AMBA Kconfig symbol in both 32-bit and
64-bit ARM architectures, move the common definition to drivers/amba
where dependent drivers will be located.

Signed-off-by: Thierry Reding <treding@nvidia.com>
diff --git a/drivers/amba/Kconfig b/drivers/amba/Kconfig
new file mode 100644
index 0000000..d1cba6a
--- /dev/null
+++ b/drivers/amba/Kconfig
@@ -0,0 +1,2 @@
+config ARM_AMBA
+	bool