ARM: OMAP2+: Probe PRCM first to probe l4_wkup with simple-pm-bus

In preparation for probing the interconnects with simple-pm-bus to
make use of genpd, we need to probe the always-on PRCM first for the
clocks needed by l4_wkup instance.

Signed-off-by: Tony Lindgren <tony@atomide.com>