drivers: spi: smartbond: Add async API support

This commit should deal with adding support
for asynchronous operations. It also adds
support for DMA acceleration via a Kconfig
variable (enaled by default as DMA should
be considered scales faster than the
interrupt-driven approach).

Signed-off-by: Ioannis Karachalios <ioannis.karachalios.px@renesas.com>