bus: ti-sysc: Flush posted write only after srst_udelay

Commit 34539b44 ("bus: ti-sysc: Flush posted write on enable before
reset") caused a regression reproducable on omap4 duovero where the ISS
target module can produce interconnect errors on boot. Turns out the
registers are not accessible until after a delay for devices needing
a ti,sysc-delay-us value.

Let's fix this by flushing the posted write only after the reset delay.
We do flushing also for ti,sysc-delay-us using devices as that should
trigger an interconnect error if the delay is not properly configured.

Let's also add some comments while at it.

Fixes: 34539b44

 ("bus: ti-sysc: Flush posted write on enable before reset")
Cc: stable@vger.kernel.org
Signed-off-by: Tony Lindgren <tony@atomide.com>