You could just install the 'firmware.tcz' extension, as the file you mentioned is included in it.
Otherwise if you already have the file at hand, you could create your very own private extension: You need the 'squashfs-tools-4.x.tcz' extension, and then perform something like:
mkdir -p /tmp/my_ext/usr/local/lib/firmware
sudo chown -R root:root /tmp/my_ext/
sudo cp -p rt2870.bin /tmp/my_ext/usr/local/lib/firmware
mksquashfs /tmp/my_ext my_ext.tcz
Then ensure that 'my_ext.tcz' is copied to your extensions directory (e.g. via cp my_ext.tcz $( cat /opt/.tce_dir)/optional) and included in the extension list for boot time (e.g. via echo my_ext.tcz >> $( cat /opt/.tce_dir)/onboot.lst)