drivers/gpio: stm32: Set SWJ higher prio vs device pin configuration

Serial Wire JTAG configuration option is made available
under condition that SPI_3 was not enabled on SOC_STM32F103XE.
Besides being obsolete there are various other potential conflicts
with other periphals, and it is not possible to explicit them all.

To make it more coherent remove such condition, assume that user
needs to take care of such pin conflict and express SWJ as having
precedence over peripheral devices pin configuration.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>