riscv: time: Add missing __iomem in get_cycles() and get_cycles_hi()

riscv: time: Add missing __iomem in get_cycles() and get_cycles_hi()

__iomem is missing while calling readl_relaxed() in get_cycles() and
get_cycles_hi() and sparse complains.

Add __iomem to silence the sparse warnings.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202607160619.14G8GHp5-lkp@intel.com/
Signed-off-by: Nam Cao <namcao@linutronix.de>
Link: https://patch.msgid.link/20260716053319.2178937-1-namcao@linutronix.de
Signed-off-by: Paul Walmsley <pjw@kernel.org>