No support for 512Mb SDRAM (10b row length)

This should be possible at a cost of 2KB code size increase:

Change this to 512:

https://github.com/xmos/lib_sdram/blob/master/lib_sdram/src/io.S#L235

Double the length of this unrolled loop:

https://github.com/xmos/lib_sdram/blob/master/lib_sdram/src/sdram_block_read_body_xs2.inc
