Sector size definitions to index.json/STM32WB5xx

Adding manually the sector sizes now. The reference manual states
these chips have a fairly nice memory map as sectors are evenly sized
and fairly small. Quote from reference manual;

"The Flash memory is organized as follows:
• A main memory block containing 256 pages of 4 KB, each page with eight rows of
512 bytes."

https://www.st.com/content/ccc/resource/technical/document/reference_manual/group0/83/cf/94/7a/35/a9/43/58/DM00318631/files/DM00318631.pdf/jcr:content/translations/en.DM00318631.pdf
- chapter 3.3.1 Flash memory organization
- internal flash starts at address 0x0800 0000

Author: Janne Kiiskila