clocksource/drivers/ingenic-ost: Add support for the JZ4760B

The OST in the JZ4760B SoC works exactly the same as in the JZ4770. But
since the JZ4760B is older, its Device Tree string does not fall back to
the JZ4770 one; so add support for the JZ4760B compatible string here.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20210308212302.10288-3-paul@crapouillou.net