Create a new configuration file: at91-sama5_common.h. Which includes the
configurations that reused by all SAMA5 chips.

at91-sama5_common.h includes:
- hw macros (clock, text_base and etc.)
- default commands.
- BOOTARGS
- U-Boot common configs.
NOTE: NOR flash definition should be put before including the common header.

For sama5d3-xplained:
- add CMD_SETEXPR

For sama5d3xek:
- add CMD_SETEXPR
- change CONFIG_SYS_MALLOC_LEN to (4*1024*1024)

Signed-off-by: Josh Wu <josh.wu@atmel.com>