pc80/i8254: Add speaker beep function

Some platforms have an onboard speaker which could be used as an
indicator of successful boot or critical error, e.g. in die_notify
function. The function assumes that SPKR GPIO is properly configured
by the platform code.

Signed-off-by: Michał Żygowski <michal.zygowski@3mdeb.com>
Change-Id: I8189b3462bb5140af352fa786db3a6a2a45076f2
Reviewed-on: https://review.coreboot.org/c/coreboot/+/68100
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Krystian Hebel <krystian.hebel@3mdeb.com>

src/include/pc80/i8254.h

@@ -43,5 +43,6 @@
 
 void setup_i8254(void);
 unsigned long calibrate_tsc_with_pit(void);
+void beep(unsigned int frequency_hz, unsigned int duration_msec);
 
 #endif /* PC80_I8254_H */