clk: Add clock driver for the RISC-V RPMI clock service group

clk: Add clock driver for the RISC-V RPMI clock service group

The RPMI specification defines a clock service group which can be
accessed via SBI MPXY extension or dedicated S-mode RPMI transport.

Add mailbox client based clock driver for the RISC-V RPMI clock
service group.

Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Co-developed-by: Anup Patel <apatel@ventanamicro.com>
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Signed-off-by: Rahul Pathak <rpathak@ventanamicro.com>
Link: https://lore.kernel.org/r/20250818040920.272664-11-apatel@ventanamicro.com
[pjw@kernel.org: converted rpmi_clkrate_u64 macro to a function; replaced bare constant with a macro]
Signed-off-by: Paul Walmsley <pjw@kernel.org>