pwm: atmel-tcb: Cache clock rates and mark chip as atomic

pwm: atmel-tcb: Cache clock rates and mark chip as atomic

atmel_tcb_pwm_apply() holds tcbpwmc->lock as a spinlock via
guard(spinlock)() and then calls atmel_tcb_pwm_config(), which calls
clk_get_rate() twice. clk_get_rate() acquires clk_prepare_lock (a
mutex), so this is a sleep-in-atomic-context violation.

On CONFIG_DEBUG_ATOMIC_SLEEP kernels every pwm_apply_state() that
enables or reconfigures the PWM triggers a "BUG: sleeping function
called from invalid context" warning.

Acquire exclusive control over the clock rates with
clk_rate_exclusive_get() at probe time and cache the rates in struct
atmel_tcb_pwm_chip, then read the cached rates from
atmel_tcb_pwm_config(). This keeps the spinlock-based mutual exclusion
introduced in commit 37f7707077f5 ("pwm: atmel-tcb: Fix race condition
and convert to guards") and removes the sleeping calls from the atomic
section.

With no sleeping calls left in .apply() and the regmap-mmio bus already
running with fast_io=true, also mark the chip as atomic so consumers
can use pwm_apply_atomic() from atomic context.

Fixes: 37f7707077f5 ("pwm: atmel-tcb: Fix race condition and convert to guards")
Signed-off-by: Sangyun Kim <sangyun.kim@snu.ac.kr>
Link: https://patch.msgid.link/20260419080838.3192357-1-sangyun.kim@snu.ac.kr
[ukleinek: Ensure .clk is enabled before calling clk_get_rate on it.]
Signed-off-by: Uwe Kleine-König <ukleinek@kernel.org>