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

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>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>