pmdomain: ti: Move and add Kconfig options to the pmdomain subsystem

The TI_SCI_PM_DOMAINS Kconfig option belongs closer to its corresponding
implementation, hence let's move it from the soc subsystem to the pmdomain
subsystem.

While at it, let's also add a Kconfig option the omap_prm driver, rather
than using ARCH_OMAP2PLUS directly.

Cc: Nishanth Menon <nm@ti.com>
Cc: Santosh Shilimkar <ssantosh@kernel.org>
Cc: Tero Kristo <kristo@kernel.org>
Cc: Tony Lindgren <tony@atomide.com>
Reviewed-by: Dhruva Gole <d-gole@ti.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>