boston: Drop unused return value

The boston lowlevel_init() function zeroes the return register v0,
despite the function not being expected to return a value & that value
never being used.

Remove the redundant assignment to v0.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>

board/imgtec/boston/lowlevel_init.S

@@ -34,7 +34,6 @@ LEAF(lowlevel_init)
 	PTR_LA	a0, msg_ddr_ok
 	bal	lowlevel_display
 
-	move	v0, zero
 	jr	s0
 	END(lowlevel_init)