rtc: zynqmp: depend on HAS_IOMEM

rtc: zynqmp: depend on HAS_IOMEM

The Xilinx zynqmp RTC driver makes use of IOMEM functions like
devm_platform_ioremap_resource(), which are only available if
CONFIG_HAS_IOMEM is defined.

This causes the driver not to be enable under make ARCH=um allyesconfig,
even though it won't build.

By adding a dependency on HAS_IOMEM, the driver will not be enabled on
architectures which don't support it.

Fixes: 09ef18bcd5ac ("rtc: use devm_platform_ioremap_resource() to simplify code")
Signed-off-by: David Gow <davidgow@google.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210127035146.1523286-1-davidgow@google.com