docs: migration-guide-3.5: add changes for i.MX RT boards

Add migration guide notes for i.MX RT boards, including the following
changes:
- i.MX RT boards must now manually enable
  CONFIG_DEVICE_CONFIGURATION_DATA and CONFIG_NXP_IMX_EXTERNAL_SDRAM
  when required
- SNVS pin names have changed for i.MX RT11xx series boards

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>