All drivers implemented so far are for chips which are only available
on legacy x86 PCs. This commit moves them into a more appropriate folder,
also making the cpu/x86/drivers/ folder ready for other x86 based SoCs.
This commit implements pic_eoi(int irq) and a helper macro PIC_INT(irq).
This first checks which PICs should be 'acked' given an IRQ number, while
the macro returns the actual system interrupt number for the IRQ according
to the offset used on the PIC initialization.
This patch implements the pic_unmask_irq() helper and uses it where
applicable. This function zeros the corresponding bit from the IRQ
number in IMR register.
This patch doesn't implement the pic_mask_irq() helper since it is not
useful at this moment.
