[Open-graphics] Timing number computations

[Timothy Miller]

Check me on these, please.  I use CP to represent the clock period in
ns.  CL is the cas latency (in clock cycles)


    r2w_wait = ceil(CL)

    w2r_wait = tWTR + 1

    act2rw_wait = ceil(tRCD/CP) - 1

    w2pre_wait = ceil(tWR/CP) + 1

    r2pre_wait = ceil(CL) - 1

    pre2act_wait = ceil(tRP/CP) - 1

    act2pre_wait = ceil(tRAS/CP) - 1

    refresh2act_wait = ceil(tRFC/CP) - 1