Nokia RX-51: Use U-Boot generic position independent code

Switch from custom board specific fixup/copy code to U-Boot generic
position independent code provided by config option POSITION_INDEPENDENT.

This also slightly decrease size of u-boot.bin binary (by 52 bytes). Note
that option POSITION_INDEPENDENT increase size but not more than custom
board fixup/copy code which is being deleted (as it is not needed anymore).

Signed-off-by: Pali Rohár <pali@kernel.org>