sizes.h: add SZ_8G/SZ_16G/SZ_32G macros

Add these macros, since we can use them in drivers.

Link: https://lkml.kernel.org/r/20201229072819.11183-1-sjhuang@iluvatar.ai


Signed-off-by: Huang Shijie <sjhuang@iluvatar.ai>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>