Add W5100 driver

[robertc2000]

No description provided.

[scaprile]

Pretty close to W5500, yet it looks like merging them won't look readable.

Please rebase to get rid of those formatting changes in mongoose.c

[robertc2000]

1. It's the same approach as W5500, but there are too many differences, especially in SPI command format and registers. So, it's best to keep the drivers in separate files.
2. It's already rebased. Looks like there is a weird issue with formatting in the mongoose.c file from 'master' branch

   mongoose/mongoose.c

   Lines 6442 to 6444 in 4a164c3

   	MG_IRAM static bool mg_stm32f_write(void *addr,
   	const void *buf,
   	size_t len) {

   After rebuilding mongoose.c, the format appears to be fixed.