Add a filesystem API for stack. The initial version of
this interface is purely synchronous.

dbrownell@users.sourceforge.net:

Cleaned up, bugfixed; much simplified; added preliminary documentation.

Works with mdev given CONFIG_SYSFS_DEPRECATED; and presumably udev.

Updated SPI_IOC_MESSAGE ioctl to full spi_message semantics, supporting
groups of one or more transfers (each of which may be full duplex if
desired).

This is marked as EXPERIMENTAL with an explicit disclaimer that the API
(notably the ioctls) is subject to change.

Signed-off-by: Andrea Paterniani
Signed-off-by: David Brownell
Cc: Arnd Bergmann
Signed-off-by: Andrew Morton
Signed-off-by: Linus Torvalds