sm501: Adjust endianness of pixel value in rectangle fill

sm501: Adjust endianness of pixel value in rectangle fill

The value from twoD_foreground (which is in host endian format) must
be converted to the endianness of the framebuffer (currently always
little endian) before it can be used to perform the fill operation.

Signed-off-by: Marcus Comstedt <marcus@mc.pp.se>
Reviewed-by: BALATON Zoltan <balaton@eik.bme.hu>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>