bus: qcom_ebi2: default y if ARCH_QCOM

Since we want this external bus to be available on multi_v7 builds,
set to default ARCH_QCOM so we get it selected whenever QCOM is
enabled.

Suggested-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>