ARM: at91: sama5: change the environment address to 0x6000

ARM: at91: sama5: change the environment address to 0x6000

As sama5 board has 32k sram size, so the at91bootstrap and spl for sama5
boards is bigger than 16k (0x4000). That will overlap the U-Boot
environment. So I move environment to 0x6000. And reduce its size as
well.

Following shows the size of the spl binaries (v2015.04 vs v2015.07):

% ls v2015.04/*spi*spl.bin -l | awk '{print $5,$(NF)}'
15540 v2015.04/at91sam9n12ek_spiflash_defconfig_u-boot-spl.bin
15704 v2015.04/at91sam9x5ek_spiflash_defconfig_u-boot-spl.bin
16064 v2015.04/sama5d3xek_spiflash_defconfig_u-boot-spl.bin
16304 v2015.04/sama5d4ek_spiflash_defconfig_u-boot-spl.bin
16304 v2015.04/sama5d4_xplained_spiflash_defconfig_u-boot-spl.bin

% ls v2015.07/*spi*spl.bin -l | awk '{print $5,$(NF)}'
16136 v2015.07/at91sam9n12ek_spiflash_defconfig_u-boot-spl.bin
16300 v2015.07/at91sam9x5ek_spiflash_defconfig_u-boot-spl.bin
16664 v2015.07/sama5d3xek_spiflash_defconfig_u-boot-spl.bin
16904 v2015.07/sama5d4ek_spiflash_defconfig_u-boot-spl.bin
16904 v2015.07/sama5d4_xplained_spiflash_defconfig_u-boot-spl.bin

The gcc version is:  gcc 4.7.3 (Ubuntu/Linaro 4.7.3-12ubuntu1)

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Reviewed-by: Andreas Bießmann <andreas.devel@googlemail.com>
Acked-by: Bo Shen <voice.shen@gmail.com>