ARM: dts: aspeed: minipack: Use common dtsi

Simplify the Minipack device tree by using the common dtsi.

In addition this enables the enabling the second firmware flash, and
updates it's size from 32MB to 64MB. It also enables the eMMC device in
slot #1.

Signed-off-by: Tao Ren <taoren@fb.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>