arm: at91: add support for mini-box picosam9g45 board

Bootlog:
U-Boot SPL 2015.10-rc1-00452-g96a7ed1 (Aug 17 2015 - 10:32:21)
mci: setting clock 258000 Hz, block size 512
mci: setting clock 258000 Hz, block size 512
mci: setting clock 258000 Hz, block size 512
mci: setting clock 33024000 Hz, block size 512
reading u-boot.img
reading u-boot.img

U-Boot 2015.10-rc1-00452-g96a7ed1 (Aug 17 2015 - 10:32:21 +0000)

CPU: AT91SAM9G45
Crystal frequency:       12 MHz
CPU clock        :      400 MHz
Master clock     :  133.333 MHz
       Watchdog enabled
DRAM:  256 MiB
WARNING: Caches not enabled
MMC:   mci: 0
mci: setting clock 260416 Hz, block size 512
mci: setting clock 260416 Hz, block size 512
mci: setting clock 260416 Hz, block size 512
mci: setting clock 33333333 Hz, block size 512
reading uboot.env
In:    serial
Out:   serial
Err:   serial
Net:   macb0
Error: macb0 address not set.

Hit any key to stop autoboot:  0
U-Boot>

Signed-off-by: Erik van Luijk <evanluijk@interact.nl>
[add 'picosam9g45_defconfig' to MAINTAINERS]
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>