asm-generic: add legacy I/O header files

The dma.h, hw_irq.h, serial.h and timex.h files originally
described PC-style i8237, i8259A, i8250, i8253 and i8255 chips
as well as the VGA style text mode graphics.

Modern architectures live happily without these specific
interfaces, but a few definitions from these headers keep
getting used in common code.

The new generic headers are what most architectures use
anyway nowadays, just implementing the minimal definitions.

Signed-off-by: Remis Lima Baima <remis.developer@googlemail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

include/asm-generic/timex.h

@@ -0,0 +1,22 @@
+#ifndef __ASM_GENERIC_TIMEX_H
+#define __ASM_GENERIC_TIMEX_H
+
+/*
+ * If you have a cycle counter, return the value here.
+ */
+typedef unsigned long cycles_t;
+#ifndef get_cycles
+static inline cycles_t get_cycles(void)
+{
+	return 0;
+}
+#endif
+
+/*
+ * Architectures are encouraged to implement read_current_timer
+ * and define this in order to avoid the expensive delay loop
+ * calibration during boot.
+ */
+#undef ARCH_HAS_READ_CURRENT_TIMER
+
+#endif /* __ASM_GENERIC_TIMEX_H */