boards: atmel: samx7x add hsmci peripheral

Adds samx7x hsmci peripheral according to datasheet sections 10-1 and 14.1: https://ww1.microchip.com/downloads/aemDocuments/documents/MCU32/ProductDocuments/DataSheets/SAM-E70-S70-V70-V71-Family-Data-Sheet-DS60001527.pdf



Updates sam_e70_xplained overlay to include pin mapping to the SD card
connected to the hsmci and adds sdhc0 alias.

Signed-off-by: John Whittington <git@jbrengineering.co.uk>