The board only has SD-Card as storage. Add a barebox update handler to
flash barebox to it, as well as an nv.boot.default=mmc1 to boot from it.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

The Groboards Giant Board is a ATSAMA5D27C-D1G SiP-based SBC.
The board features a 500MHz ARM Cortex-A5 and 128MB DDR2 SDRAM in the
SiP as well as a MicroSD slot on the PCB.

barebox doesn't yet support the sama5d2 SDHCI-variant, so board support
is limited to toggling the LED and interfacing with the UART, but add
this now till the rest may follow. Device tree is based on the vendor's
device-tree available at https://github.com/Groboards/giantboard-tools

Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>