firmware: arm_scmi: Use 64-bit division for clock rate rounding

firmware: arm_scmi: Use 64-bit division for clock rate rounding

SCMI clock range descriptors report rates as 64-bit values. When handling
a range clock, scmi_clock_determine_rate() rounds the requested rate up to
the next supported step using the SCMI RATE_STEP value.

The current code uses div64_ul() for this calculation. Since div64_ul()
takes an unsigned long divisor, the 64-bit RATE_STEP value can be truncated
on 32-bit builds. In the worst case, a non-zero 64-bit step can be narrowed
to zero before the division.

Store RATE_STEP in a u64, reject a malformed zero step, and use
DIV64_U64_ROUND_UP() so the divisor is handled as a 64-bit value.

This does not change behavior for valid firmware reporting a non-zero step
that fits in unsigned long.

Tested on Xunlong Orange Pi 5 Plus / RK3588 with SCMI over SMC. SCMI
clocks probed successfully before and after the change. SCMI-backed CPU
clocks were exercised through cpufreq-dt by switching each CPU policy
between its lowest and highest available OPP.

Fixes: ecde921eb460 ("firmware: arm_scmi: Add clock determine_rate operation")
Signed-off-by: Steve Dunnagan <sdunnaga@redhat.com>
Link: https://patch.msgid.link/20260701195923.444270-1-sdunnaga@redhat.com
Signed-off-by: Sudeep Holla <sudeep.holla@kernel.org>