Allow pxa2x0_lcd driver mapping screen buffer memory cachable with
write-through map (i.e. map it without BUS_DMA_COHERENT) since
currently all DMA data transfers are memory to device only.

Disabled by default, but enabled by "options PXA2X0_LCD_WRITETHROUGH"
or setting pxa2x0_lcd_writethrough = 1 in a kernel binary.

Tested on WS003SH by me and on WS011SH by jun@, and console output speed
is improved ~three times faster than coherent (uncached) mapping.

XXX: should we have a flag like BUS_DMA_WRITETHROUGH in MI bus_dma(9)?

(tsutsui)