Fixes for PS/2 touchpad support

[jackpot51]

As the PS/2 driver is used by default in Windows prior to driver install, as well as in other operating systems with no I2C HID support, it would be helpful to have it fully functional. Draft as I work through the changes.

Tested with Windows 10 installer and Redox OS.

Going through the changes:

• Constants replace magic numbers for PS2 control registers
• Touchpad data is buffered
• KBC_STATE_TOUCHPAD is dropped
• Buffer is cleared on touchpad reset command
• kbc_on_output_empty pops from touchpad buffer only when ready to send to host
• Move keyboard buffer check to kbc_on_output_empty
• Reset PS2 bus after write transaction finishes
• Wait until next cycle to attempt touchpad read