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

arch/arm/mach-ixp4xx/Kconfig

@@ -13,7 +13,6 @@ config MACH_IXP4XX_OF
 	select I2C
 	select I2C_IOP3XX
 	select PCI
-	select TIMER_OF
 	select USE_OF
 	help
 	  Say 'Y' here to support Device Tree-based IXP4xx platforms.

drivers/clocksource/Kconfig

@@ -79,6 +79,7 @@ config IXP4XX_TIMER
 	bool "Intel XScale IXP4xx timer driver" if COMPILE_TEST
 	depends on HAS_IOMEM
 	select CLKSRC_MMIO
+	select TIMER_OF if OF
 	help
 	  Enables support for the Intel XScale IXP4xx SoC timer.