drivers: flash: silabs: replace 'depends on DMA' with 'select'

When a driver requires the usage of a low level driver subsystem
(like SPI, I2C, DMA...), it shall enable it instead of depending on it.
Also, make DMA usage depend on its DT properties being defined.

Signed-off-by: Abderrahmane JARMOUNI <git@jarmouni.me>