irqchip/ath79-misc: Fix missing prototypes warnings

ath79_misc_irq_init() was defined but unused since commit 51fa4f89
("MIPS: ath79: drop legacy IRQ code"), so it's time to drop it.

The build also warns about a missing prototype of get_c0_perfcount_int().

Remove the stale leftover function and add the missing include.

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/OSBPR01MB167032D2017645200787AAEBBC72A@OSBPR01MB1670.jpnprd01.prod.outlook.com