of: reserved_mem: Fix typo in the too-many-regions message

Minor fix for a missing preposition in the error message that appears
when there are too many reserved memory regions for the allocated array
to store.

Signed-off-by: Danny Lin <danny@kdrag0n.dev>
Link: https://lore.kernel.org/r/20200604054900.200317-1-danny@kdrag0n.dev


Signed-off-by: Rob Herring <robh@kernel.org>