arch: arm: stm32l4: Add pin config for USART1 on PB6/PB7
arch: arm: stm32l4: Add pin config for USART1 on PB6/PB7

With introduction of commit "pinmux: stm32: directly return error if
stm32_get_pin_config fails", pin configuration fails when
pins are not configured in pins[] array.
This was the case for configuration UART1 assigned on PB6/PB7.

Signed-off-by: Erwan Gouriou <de4d967452b83bb0e37525652734e7d9163b6a4c@linaro.org>

explora26/zephyr,zephyriot/zephyr,rsalveti/zephyr,finikorg/zephyr,galak/zephyr,mbolivar/zephyr,punitvara/zephyr,kraj/zephyr,fbsder/zephyr,runchip/zephyr-cc3220,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,fbsder/zephyr,aceofall/zephyr-iotos,nashif/zephyr,galak/zephyr,Vudentz/zephyr,explora26/zephyr,aceofall/zephyr-iotos,galak/zephyr,GiulianoFranchetto/zephyr,rsalveti/zephyr,runchip/zephyr-cc3220,Vudentz/zephyr,runchip/zephyr-cc3220,zephyrproject-rtos/zephyr,fbsder/zephyr,zephyriot/zephyr,aceofall/zephyr-iotos,explora26/zephyr,Vudentz/zephyr,kraj/zephyr,mbolivar/zephyr,zephyriot/zephyr,ldts/zephyr,kraj/zephyr,ldts/zephyr,galak/zephyr,zephyriot/zephyr,explora26/zephyr,ldts/zephyr,rsalveti/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,galak/zephyr,fbsder/zephyr,rsalveti/zephyr,finikorg/zephyr,ldts/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,Vudentz/zephyr,rsalveti/zephyr,nashif/zephyr,mbolivar/zephyr,kraj/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,aceofall/zephyr-iotos,nashif/zephyr,zephyriot/zephyr,mbolivar/zephyr,runchip/zephyr-cc3220,mbolivar/zephyr,runchip/zephyr-cc3220,Vudentz/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,finikorg/zephyr,finikorg/zephyr,Vudentz/zephyr,aceofall/zephyr-iotos,fbsder/zephyr,punitvara/zephyr,finikorg/zephyr,nashif/zephyr