microchip_23k640_spi_sram: initial commit

Implementation of the Microchip part 23K640, an 8K x 8 Bit SPI Static
 RAM, based on datasheet specifications.

Client protocol code appears to be valid/relevant for similar parts
 up to a full 64k x 8 Bits.  For implementing a part greater than 64k
 looks to be a simple addition of latching a third address byte for
 a total of 24 address bits.

	new file:   microchip_23k640_spi_sram.c
	new file:   microchip_23k640_spi_sram.h