board: stm32mp1: update readme

Update readme:
- list the supported SOC and change family to STM32MP15x
- add warning on OTP write and prerequisite:
  check if MAC address is not yet provisioned.
- Use filesize for mmc write command (avoid to write all partition
  with ${partsize}). ${filesize} and ${partsize} are set by previous
  load command.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>

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