Move DQS related code into a DQSGroup elaboratable

[jeanthom]

Better primitive abstraction = able to do more tests

[jeanthom]

Here's a draft:

class DQSGroup(Elaboratable):
    def __init__(self, *, dqs, dq, dm, rdly_csr):
        # Pads
        self.dqs = dqs
        self.dq = dq
        self.dm = dm

        # CSR
        self.rdly_csr = rdly_csr

        # DQSBUFM informations
        self.burstdet = Signal()
        self.datavalid = Signal()

        # Timing signals needed here

        # Data IO
        self.din = Signal(8*4)
        self.dout = Signal(8*4)
        self.dmask = Signal(4)

    def elaborate(self, platform):
        m = Module()
        ...
        return m