fix(arm): move console flush/switch in common function

There are some CI configs which apply patch on the fly to test some
unusual test scenarios. After commit c864af98

 there is one patch which
does not apply cleanly into arm_bl31_plat_runtime_setup().

To fix this issue move console flush/switch into the caller of this
function.

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: I4116044d53bef349a707c977cf26d1df65200045