cpuidle: Factor-out power domain related code from PSCI domain driver

The generic power domain related code in PSCI domain driver is largely
independent of PSCI and can be shared with RISC-V SBI domain driver
hence we factor-out this code into dt_idle_genpd.c and dt_idle_genpd.h.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>