risc-v: Use generic io.h helpers for nommu

Without MMU support PCI_IOBASE is left undefined because PCI_IO_END is
VMEMMAP_START. Nevertheless the in*()/out*() helper macros are left
defined with uses of PCI_IOBASE.

At the moment this only compiles because asm-generic/io.h defines
PCI_IOBASE as 0 if it is undefined and so at macro expansion PCI_IOBASE
is defined. This leads to compilation errors when asm-generic/io.h is
changed to leave PCI_IOBASE undefined.  More importantly it is currently
broken at runtime, as accessing a fixed I/O port number of an ISA device
on NOMMU RISC-V would turn into a NULL pointer dereference.

Instead only define the in*()/out*() helper macros with MMU support and
fall back to the asm-generic/io.h helper stubs otherwise.

Signed-off-by: Niklas Schnelle <niklas@komani.de>

arch/riscv/include/asm/io.h

@@ -23,12 +23,12 @@
 #include <asm/mmio.h>
 
 /*
- *  I/O port access constants.
+ *  I/O port access constants. Without MMU support leave PCI_IOBASE undefined
+ *  and fall back to generic stubs for I/O access routines.
  */
 #ifdef CONFIG_MMU
 #define IO_SPACE_LIMIT		(PCI_IO_SIZE - 1)
 #define PCI_IOBASE		((void __iomem *)PCI_IO_START)
-#endif /* CONFIG_MMU */
 
 /*
  * Emulation routines for the port-mapped IO space used by some PCI drivers.
@@ -145,6 +145,7 @@ __io_writes_outs(writes, u64, q, __io_bw(), __io_aw())
 __io_writes_outs(outs, u64, q, __io_pbr(), __io_paw())
 #define outsq(addr, buffer, count) __outsq((void __iomem *)addr, buffer, count)
 #endif
+#endif /* CONFIG_MMU */
 
 #include <asm-generic/io.h>