[POWERPC] Virtual DMA support for floppy driver for new powerpc architecture

During ppc64+ppc merge virtual DMA code for floppy driver was not
ported.  This patch restores virtual DMA support for floppy in new
powerpc target.

It is necessary at least on Pegasos and AmigaOne machines for the
floppy drive to function.  ISA DMA controller works incorrectly there
due to its addressing limitations.

Virtual DMA mode is activated by floppy=nodma option passed to the
kernel (or module).  There's no automatic switch like on i386.

Signed-off-by: Pavel Fedin <sonic_amiga@rambler.ru>
Signed-off-by: Paul Mackerras <paulus@samba.org>