The 315-5364 is the Bus Arbiter/68K address decoder found in later Model 1 Mega Drives. In essence, it is simply an integration of the 315-5308 and the 315-5345 into a single chip, presumably to make the board easier and cheaper to manufacture.
It's primary purpose is to act as a bridge between the 68k and Z80, providing the Z80's clock and reset lines, as well as chip selects for it's various periphery. It also holds the Z80's bank register used to select the appropriate 68k bank, whose address this chip puts on the bus during free cycles.
As the 315-5364 does not have enough pins for the complete Z80 address or data bus, it uses the 315-5309 to handle additional Z80 bridge duties, namely bridging of the lower address bits and the data bus between the 68k and Z80.