arm64/arch_timer: Fix MMIO byteswap

The readl_relaxed() to __raw_readl() change meant to loose the
instrumentation, but also (inadvertently) lost the byteswap.

Fixes: 24ee7607 ("arm64/arch_timer: Provide noinstr sched_clock_read() functions")
Reported-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lkml.kernel.org/r/20230606080614.GB905437@hirez.programming.kicks-ass.net