doc: loaders_io: Add GP_IOSubIO() documentation

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
tests: loaders: IO: Much more tests.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
loaders: IO; Fix IOMem and IOSubIO seeks

Signed-off-by: Cyril Hrubis <metan@ucw.cz>