boards: frdm_rw612: Document SRAM partitions

Add comment to DTS file about SRAM partitions similar to the RTXXX
series has comments.

Add also a doc section to the frdm_rw612 about this.

Also fix the section hierarchy of the frdm_rw612 doc, the header levels
were wrong since the wifi and bluetooth, and reference sections were
under the debugging section.
Group all the wireless connectivity info together.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>