drivers: spi: spi_nrfx_spim: fix !LEGACY_INCLUDE_PATH

Add missing zephyr/ prefix to fix CONFIG_LEGACY_INCLUDE_PATH=n build.

Fixes: d64dc771 ("drivers: spi: spi_nrfx_spim: utilize EasyDMA property
  from devicetree")

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>