mv-common.h: fix DRAM banks configuration

The asm/arch/config.h header define CONFIG_NR_DRAM_BANKS_MAX, which is
needed to configure DRAM banks.

This patch move the asm/arch/config.h header inclusion above the DRAM
banks configuration.

Additionally this patch fix a typo.

Signed-off-by: Simon Guinot <sguinot@lacie.com>