stm32: quadspi overhaul documentation

Provide all doxygen framework boilerplate.
Provide a very initial stub .c file to ensure that heirarchical headers
are pulled in properly.

lib/stm32/common/quadspi_common_v1.c

@@ -0,0 +1,12 @@
+
+#include <libopencm3/stm32/quadspi.h>
+
+void quadspi_enable(void)
+{
+	QUADSPI_CR |= QUADSPI_CR_EN;
+}
+
+void quadspi_disable(void)
+{
+	QUADSPI_CR &= ~QUADSPI_CR_EN;
+}