OpenThread: Change SETTINGS_CONFIG_PAGE_SIZE to target specific value

- Changed define for SETTINGS_CONFIG_PAGE_SIZE from a hard coded value
  to reference build system generated FLASH_ERASE_BLOCK_SIZE. This value
  comes from 'erase-block-size' found in the dtsi file of devices.
- Modified nrf52840.dtsi to include definition for 'erase-block-size'

Fixes #7107

Signed-off-by: David Leach <david.leach@nxp.com>