net: stmmac: Fix sub-second increment

In fine adjustement mode, which is the current default, the sub-second
    increment register is the number of nanoseconds that will be added to
    the clock when the accumulator overflows. At each clock cycle, the
    value of the addend register is added to the accumulator.
    Currently, we use 20ns = 1e09ns / 50MHz as this value whatever the
    frequency of the ptp clock actually is.
    The adjustment is then done on the addend register, only incrementing
    every X clock cycles X being the ratio between 50MHz and ptp_clock_rate
    (addend = 2^32 * 50MHz/ptp_clock_rate).
    This causes the following issues :
    - In case the frequency of the ptp clock is inferior or equal to 50MHz,
      the addend value calculation will overflow and the default
      addend value will be set to 0, causing the clock to not work at
      all. (For instance, for ptp_clock_rate = 50MHz, addend = 2^32).
    - The resolution of the timestamping clock is limited to 20ns while it
      is not needed, thus limiting the accuracy of the timestamping to
      20ns.

    Fix this by setting sub-second increment to 2e09ns / ptp_clock_rate.
    It will allow to reach the minimum possible frequency for
    ptp_clk_ref, which is 5MHz for GMII 1000Mps Full-Duplex by setting the
    sub-second-increment to a higher value. For instance, for 25MHz, it
    gives ssinc = 80ns and default_addend = 2^31.
    It will also allow to use a lower value for sub-second-increment, thus
    improving the timestamping accuracy with frequencies higher than
    100MHz, for instance, for 200MHz, ssinc = 10ns and default_addend =
    2^31.

v1->v2:
 - Remove modifications to the calculation of default addend, which broke
 compatibility with clock frequencies for which 2000000000 / ptp_clk_freq
 is not an integer.
 - Modify description according to discussions.

Signed-off-by: Julien Beraud <julien.beraud@orolia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>