include/config_fallbacks.h: add default for CONFIG_SYS_MAXARGS

CONFIG_SYS_MAXARGS contains the maximum number of arguments accepted
by U-Boot commands. Since the vast majority of the platforms define it
to 16, it makes sense to have a default definition to 16, which will
allow to remove this definition from a significant number of
platforms.

It will allow to remove the default definition from 216 platform .h
files, leaving only 56 platforms with non-default values (15, 24, 32,
48, 64, 96, 128 or 256).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Tom Rini <trini@konsulko.com>