clocksource/drivers/ixp4xx: Select TIMER_OF when needed

Compile-testing the ixp4xx timer with CONFIG_OF enabled but
CONFIG_TIMER_OF disabled leads to a harmless warning:

arm-linux-gnueabi-ld: warning: orphan section `__timer_of_table' from `drivers/clocksource/timer-ixp4xx.o' being placed in section `__timer_of_table'

Move the select statement from the platform code into the driver
so it always gets enabled in configurations that rely on it.

Fixes: 40df14cc ("clocksource/drivers/ixp4xx: Add OF initialization support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20210103135955.3808976-1-arnd@kernel.org