PPC4xx: ALPR: constify FPGA code

The ALPR custom FPGA code was missed by commit e6a857da "fpga:
constify to fix build warning" resulting in such warnings:

fpga.c:226: warning: initialization from incompatible pointer type

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Stefan Roese <sr@denx.de>
Acked-by: Stefan Roese <sr@denx.de>