kconfig: Deprecate BOOTLOADER_SRAM_SIZE

This Kconfig is seemingly unused with exception to a single board,
whose maintainers have been unresponsive for a long time. It is
not useful to any other board and can be set in dts, therefore
deprecate it and schedule removal after the Zephyr 3.7 release

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>