arm64: sysreg: Fix use of XZR in write_sysreg_s

Commit 8a71f0c656 ("arm64: sysreg: replace open-coded mrs_s/msr_s with
{read,write}_sysreg_s") introduced a write_sysreg_s macro for writing
to system registers that are not supported by binutils.

Unfortunately, this was implemented with the wrong template (%0 vs %x0),
so in the case that we are writing a constant 0, we will generate
invalid instruction syntax and bail with a cryptic assembler error:

  | Error: constant expression required

This patch fixes the template.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/sysreg.h

@@ -286,7 +286,7 @@ asm(
 
 #define write_sysreg_s(v, r) do {					\
 	u64 __val = (u64)v;						\
-	asm volatile("msr_s " __stringify(r) ", %0" : : "rZ" (__val));	\
+	asm volatile("msr_s " __stringify(r) ", %x0" : : "rZ" (__val));	\
 } while (0)
 
 static inline void config_sctlr_el1(u32 clear, u32 set)