directfb: Fix ps2mouse driver availability

Closes #553.

The ps2mouse driver is miss named because it's not only a driver for
ps2 mice, but also usb mice. So it's not only limited to PC architecture
(x86 & x86-64).

Signed-off-by: Lionel Landwerlin <lionel.landwerlin@openwide.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>