[stm32] spektrum: selectable bind pin
[stm32] hack to make UART4/5 work with spektrum macros

Since UART4/5 is already defined in libopencm3 the macros don't work for those.
(Compared to e.g. UART1 which is actually USART1 in libopencm3).

So just define RADIO_CONTROL_SPEKTRUM_[PRIMARY|SECONDARY]_PORT to SPEKTRUM_UARTx instead of UARTx.
This makes it at least possible to configure the secondary spektrum port (UART5 on Lisa) properly.