meson/gxl: Add support for SHA256 DMA engine

In order to configure and boot SCP, BL31 has to compute and send
the SHA-256 of the firmware data via scpi. Luckily Amlogic GXL SOC
has a DMA facility that could be used to offload SHA-256
computations. This adds basic support of this hardware SHA-256
engine.

Signed-off-by: Remi Pommarel <repk@triplefau.lt>