clocksource/drivers/timer-sp804: Fix an Oops when read_current_timer is called on ARM32 platforms where the SP804 is not registered as the sched_clock.

In the Linux kernel, the following vulnerability has been resolved:

clocksource/drivers/timer-sp804: Fix an Oops when read_current_timer is called on ARM32 platforms where the SP804 is not registered as the sched_clock.

On SP804, the delay timer shares the same clkevt instance with
sched_clock. On some platforms, when
sp804_clocksource_and_sched_clock_init is called with use_sched_clock
not set to 1, sched_clkevt is not properly initialized. However,
sp804_register_delay_timer is invoked unconditionally, and
read_current_timer() subsequently calls sp804_read on an uninitialized
sched_clkevt, leading to a kernel Oops when accessing
sched_clkevt->value.

Declare a dedicated clkevt instance exclusively for delay timer,
instead of sharing the same clkevt with sched_clock. This ensures
that read_current_timer continues to work correctly regardless of
whether SP804 is selected as the sched_clock.