board: at91: sama7g54_curiosity: Add initial board support

Add initial support for SAMA7G54 Curiosity board.

Hardware:
SoC: SAMA7G54D2G SiP 1000 MHz
DRAM: DDR3 256 MiB
PMIC: MCP16502
Debug: UART3
Flash: QSPI NOR 8 MiB, SLC NAND 512 MiB
M.2 slot for wireless
Mikrobus connectors x 2
SD-Card connectors x 1
USB 2.0 x 3

Signed-off-by: Mihai Sain <mihai.sain@microchip.com>