serial: drop redundant depends on

SANDBOX_SERIAL depends on SANDBOX, and SANDBOX selects DM.
So, "SANDBOX_SERIAL depends on DM" is redundant.

Likewise, UNIPHIER_SERIAL depends on ARCH_UNIPHIER, and
ARCH_UNIPHIER selects DM_SERIAL.
So, "UNIPHIER_SERIAL depends on DM_SERIAL" is redundant.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Simon Glass <sjg@chromium.org>