mainboard/ms7721: Add MSI MS-7721 (FM2-A57MA-E35)

Adds support for the MSI MS-7721 (FM2-A75MA-E35) motherboard.

Tested by building coreboot with:
 - VGA bios (needed for onboard video)
 - XHCI firmware
 - SeaBIOS payload

CPU: AMD A8-6500 APU
RAM: 2x 2GB Samsung M378B5673EH1

Confirmed booting using:
 - USB stick with Arch Linux (kernel 4.7.5)
 - Gentoo live CD from SATA dvd drive
 - Gentoo installation from SATA harddisk (kernel 4.4.26)

Change-Id: I757e011de01ca9f340fd524b10e7fa3f291d53e3
Signed-off-by: Renze Nicolai <renze@rnplus.nl>
Reviewed-on: https://review.coreboot.org/17495
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
7 files changed