x86: Rename pcat_ to i8254 and i8259 accordingly

Rename pcat_timer.c to i8254.c and pcat_interrupts.c to i8259.c,
to match their header file names (i8254.h and i8259.h).

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>