scripts/spelling.txt: add "fimware" misspelling

A few instances of "fimware" instead of "firmware" were found.  Fix
these and add it to the spelling.txt file.

Signed-off-by: Kees Cook <keescook@chromium.org>
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>