targets/nexys4ddr: fix sdcard clocker initialization

[gsomlo]

Following the new clocker from enjoy-digital/litesdcard@2d4230c this is my attempt to get the nexys4ddr to successfully build using --with-sdcard once again. Based on my experiments, one would now have to go with at least

sdclk 20
sdinit
sdtest 8

from the LiteX bios command line. Anything below 20 and initialization fails for me (rocket cpu at sys_clk_freq=67e6 -- the maximum that will pass timing for rocket when --with-ethernet is also selected).

@enjoy-digital PTAL at the new add_period_constraint() statements in particular, as I'm not sure period_ns(self.sys_clk_freq)) is the right value for the constraints (in fact I'm not sure I understand how that part works at all, so if you can explain or point me to where I can figure it out more easily, I'd appreciate that too :) )

[enjoy-digital]

Thanks. The PR is fine and we can use the add_period_constraint() you added. Initially, the SD Card clock is initialized to 10MHz, but we change it dynamically to increase it after initialization has been done so we need to the constraints to the maximum frequency we are going to use. Here using sys_clk_freq seems fine.

It's possible to the S7MMCM is configuring the MMCM multipliers/dividers differently than what we were doing previously in SDClockerS7. So the current update we are doing on the dividers is probably not correct, which would explain why it only works with sdclk 20 We should probably reuse similar code than what is used by Antmicro for the Linux driver in the BIOS. (I will look at that).