i8042: Use functions to handle register access

At present the register access in kbd_reset() is quite primitive. This makes
it hard to follow.

Create functions to read and write data, both to a single register, and via
the command/data approach.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-on: Intel Crown Bay and QEMU
Tested-by: Bin Meng <bmeng.cn@gmail.com>