ARC: thread_info.h: correct two typos in a comment

Fix typos of "separately" and "remains".

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Suggested-by: Matthew Wilcox <willy@infradead.org> # "remains"
Cc: Vineet Gupta <vgupta@kernel.org>
Cc: linux-snps-arc@lists.infradead.org
Signed-off-by: Vineet Gupta <vineetg@rivosinc.com>