[MTD] [NAND] pxa3xx_nand: added some 'const' annotations to the exported API

This patch marks some attributes as 'const' which are set only once and
never be modified by the driver.  There are some changes in parameter
list and variable declarations too which mark them as 'const'.

Signed-off-by: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>