Add pread and pwrite functions.

Split read and write functions to save some space.
The functions pread and pwrite saves and sets the file
position to a given offset and restore them afterwards.

This also makes the nandtest command use these function
which is necessary to not break compilation for the nandtest
command.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>