drivers: spi: stm32: don't include zephyr/arch/cache.h

Remove inclusion of zephyr_arch/cache.h header file from STM32 SPI
driver. This header file is included by zephyr/cache.h if applicable
(e.g. CONFIG_ARCH_CACHE is enabled) and should not be used when
CONFIG_EXTERNAL_CACHE is enabled.

Signed-off-by: Etienne Carriere <etienne.carriere@st.com>