firmware: arm_scmi: update rate_discrete in clock_describe_rates_get

firmware: arm_scmi: update rate_discrete in clock_describe_rates_get

The boolean rate_discrete needs to be assigned to clk->rate_discrete,
so that clock driver can distinguish between the continuous range and
discrete rates. It uses this in scmi_clk_round_rate could get the
rounded value if it's a continuous range.

Fixes: 5f6c6430e904 ("firmware: arm_scmi: add initial support for clock protocol")
Signed-off-by: Peng Fan <peng.fan@nxp.com>
[sudeep.holla: updated commit message]
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>