net: stmmac: Refactor EST implementation

Refactor EST implementation by moving common code for DWMAC4 and
DWXGMAC IPs into a separate EST module. EST implementation for DWMAC4
and DWXGMAC differs only for CSR base address, PTOV field offset
width, and PTOV clock multiplier value.

Thanks, Serge Semin and Jakub Kicinski for the suggestions on
refactoring EST implementation into a separate EST module.

Signed-off-by: Rohan G Thomas <rohan.g.thomas@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20231201055252.1302-3-rohan.g.thomas@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>