You can activate the 16-bit RGB Mode with 8-bit serial communication by tying the IM[3:0] pins to VDD.
You'll use DB[17:3], DB[11:1] for your 5-6-5 data (D[17:13] Red, D[11:6] Green, D[5:1] Blue). The unused D0 & D12 pins must be tied to ground.
The remaining bits required to set up the RGB interface (DPI, RCM) can be accessed via SPI through the following connections to your microcontroller:
SDO SDI (MISO)
SDI (SDA) SDO (MOSI)
DE GPIO (or VDD/VSS if not used)
The unused RD pin should be fixed to VDD or VSS.