arm64: why set SECTION_SIZE_BITS to 1G size?

[Xishi Qiu]

Hi everyone:
arm64:
SECTION_SIZE_BITS 30 -----1G

The memory hotplug(add_memory -->check_hotplug_memory_range) 
must be aligned with section.So I can not add mem with 64M ...
Can I modify the SECTION_SIZE_BITS to 26?

Thanks,
Xishi Qiu