bios/sdram: Use an LFSR to speed up pseudo-random number generation

bios/sdram: Use an LFSR to speed up pseudo-random number generation

This speeds up the memory test by an order of magnitude, esp. on
cores without a hardware multiplier by getting rid of the
multiplication in the loop.

The LFSR implementation comes from microwatt's simple_random test
project.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>