spi: stm32: deduplicate code creating SPI instances

Create new instances of SPI using single STM32_SPI_INIT() macro
invocation, similar as it is done for STM32 UART driver. Add also
implicit '#ifdef CONFIG_SPI_id' check, so it further reduces required
lines of code for each SPI instance definition.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>