liblitedram/sdram: Add option to disable cdelay()

liblitedram/sdram: Add option to disable cdelay()

When running in sim, those delays can take a *long* time, which
isn't always necessary with the simulated litedram PHY.

This allows system.h to optionally set CONFIG_SIM_DISABLE_DELAYS
which  causes cdelay to do nothing.

This is especially useful when using a verilated litedram inside
a bigger/slower simulated design as to not spend a huge amount
of time going through the initializations.

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