arc: Cleanup and audit usage of <config.h>

We need to include <config.h> directly when a file needs to have
something such as CFG_SYS_SDRAM_SIZE referenced as this file is not
automatically globally included and is most commonly indirectly included
via common.h.  Remove most cases of arc including config.h directly, but
add it where needed. Further clean up the tb100 board config.h file so
that we don't rely on config.h being included there for a value used in
a single place.

Signed-off-by: Tom Rini <trini@konsulko.com>
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Cc: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Cc: uboot-snps-arc@synopsys.com