[nrf fromtree] kernel: Guard SYS_CLOCK_HW_CYCLES_PER_SEC to avoid spurious empty macro

If SYS_CLOCK_EXISTS is not enabled, then the SYS_CLOCK_HW_CYCLES_PER_SEC
still gets created, but with no value. This causes the code generation
in misc/generated/CMakeLists.txt to create an empty assembly macro:

`.equ  CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC,`

which then causes a build error.

Disable SYS_CLOCK_HW_CYCLES_PER_SEC entirely when SYS_CLOCK_EXISTS is
disabled to fix this.

This is a follow-up to 03f46db.

Signed-off-by: Carles Cufi <[email protected]>
(cherry picked from commit 61cdcb3)

Author: carlescufi

kernel/Kconfig

@@ -829,6 +829,7 @@ config SYS_CLOCK_TICKS_PER_SEC
 config SYS_CLOCK_HW_CYCLES_PER_SEC
 	int "System clock's h/w timer frequency"
 	default 0 if TIMER_READS_ITS_FREQUENCY_AT_RUNTIME
+	depends on SYS_CLOCK_EXISTS
 	help
 	  This option specifies the frequency of the hardware timer used for the
 	  system clock (in Hz). This option is set by the SOC's or board's Kconfig file