target/cortex_m: prevent asserting reset if examine is deferred

target/cortex_m: prevent asserting reset if examine is deferred

In a corner case when debug_ap is not available,
cortex_m_assert_reset() asserts reset to restore
communication with the target.

Prevent to do so on targets with defer_examine,
as such targets need some special handling to enable them
after reset anyway.

The change makes possible to handle a multicore Cortex-M SoC with
an auxiliary Cortex-M core(s) switched of by default
even with 'reset_config srst_gates_jtag'

Change-Id: I8cec7a816423e588d5e2e4f7904c81c776eddc42
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/8097
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>