phy/usddrphy: simplify tCK reference by using a specific ODELAYE3 in FIXED mode and keep the PHY in reset by default.

Keeping the PHY in reset by default was not possible previously due to the ODELAYE3 configuration
that was lost on reset. Using a specific ODELAYE3 in FIXED mode allow keeping the PHY in reset at
startup.

Allow adding a delay on Clock/Commands. Delay is manual for now but could be
automated in the future if useful (as done on 7-series/Ultrascale(+)).

One some hardware, forcing cl or/and cwl to non-default values can provide
better results.

common: rename get_cl_cw function to get_default_cl_cwl (and provide retro-compat) and add get_default_cl, get_default_cwl functions.

modules: add MT48LC32M8 SDR module

Required on OrangeCrab that has LDM/UDM swapped.

Now that we are restricting cmd/clk scan in liblitedram, cmd_latency=0 seems
to be workin for all configurations.

Only covers unit-tests for now, Verilator simulation and benchmarks still need to be adapted.

bench/xcu1525: use specific output_dir per channel (to allow // build of bitstreams for the different channels).

ecp5ddrphy: Fix DELAYF initial value

I've seen intermittent failures with recent builds. I suspect it is
related to the DELAYF primitive not being correctly loaded with an
initial value. Holding LOADN LOW ensures a value is correctly
loaded.