time: Replace hard-coded values with macro

This is more descriptive than the comment and allows for changing
values, such as using a chip with a different clock frequency.

The resulting binary is identical.

Signed-off-by: Tim Crawford <tcrawford@system76.com>

src/ec/ite/ec.mk

@@ -11,6 +11,9 @@ ec-y += ps2.c
 ec-y += signature.c
 ec-y += wuc.c
 
+# Chip clock frequency: 9.2 MHz
+CFLAGS += -DCONFIG_CLOCK_FREQ_KHZ=9200
+
 ifeq ($(CONFIG_EC_ITE_IT8587E), y)
 CFLAGS+=-DCONFIG_EC_ITE_IT8587E=1
 # SRAM is 4096 bytes, but SRAM at address 2048 is used for scratch ROM