How to arrange the position of this legend

Question

As shown in the map below, how can I place the legend to the last cell in the grid?

The code I used is

psp1 <-   tm_shape(province) + 
  tm_borders(col = 'black') + 
  tm_shape(county) + 
  tm_polygons(col = 'estimate', title = 'Changes in %', style = 'fixed', palette = brewer.pal(n = 6, name = 'Spectral'), 
              breaks = c(-15, -10, -5, 0, 5, 10, 15), legend.hist = F) + 
  tm_facets('warming', ncol = 2) + 
  tm_shape(province) + 
  tm_borders(col = 'black') + 
  tm_compass(north = 0, type = 'arrow', show.labels =0, position = c('right','top')) + 
  tm_layout(legend.format = list(fun = function(x) formatC(x, digits = 1, format = "f")),
            fontface = 'bold',
            legend.text.size = 1.3,
            legend.width = 0.2,
            legend.title.size = 1.5,
            panel.label.size = 1.5,
            panel.label.fontface = 'bold')

The data can be found from here. Thanks.