ARM: DRA7x/AM57xx: Get rid of CONFIG_AM57XX

CONFIG_AM57XX is just an unnecessary macro that is redundant given So,
remove the same instead of spreading through out the u-boot source
code and getting in the way to maintain common code for DRA7x family.

Acked-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>