clk: versatile: Rename ICST to CLK_ICST

For consistency, prefix the ICST config option with CLK as all other
clock source drivers have.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Stephen Boyd <sboyd@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20210901180833.4558932d@endymion


Signed-off-by: Stephen Boyd <sboyd@kernel.org>