drivers: stm32: SPI: SPI nocache buffers can be in CONFIG_NOCACHE_MEMORY

CONFIG_NOCACHE_MEMORY is a valid way of declaring buffers in
nocache regions. Consider them valid in the stm32 SPI driver
nocache check. Also, don't check NULL buffers as the SPI
interface states that such buffers will result in sending
zeroes.

Signed-off-by: Daniel Gaston Ochoa <dgastonochoa@gmail.com>