MAINTAINERS: Update section for Ux500 clock drivers

There's no longer any need host a tree solely to serve changes for the
Ux500 clock driver, thus drop this from the corresponding section and use
the common clk tree instead.

Moreover, let's also add the generic linux-clk mailing list and rename the
section header.

Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lkml.kernel.org/r/20191121100726.17725-1-ulf.hansson@linaro.org
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>