arch/arm64 Fix the SP unalignment issues

zynq-mpsoc and zcu111 borad was broken by #15437, because #15437 changes
ARM64_CONTEXT_REGS from 36 to 37, resulting in the stack no longer being
16-byte aligned which appears to violate the ARMv8-A architecture's
requirement for 16-byte stack alignment. this commit changes ARM64_CONTEXT_REGS
to 38 to fix this issues.

arch/arm64/include/irq.h

@@ -154,10 +154,11 @@
 #define REG_SP_EL0          (34)
 #define REG_EXE_DEPTH       (35)
 #define REG_SCTLR_EL1       (36)
+#define REG_RESERVED        (37)
 
 /* In Armv8-A Architecture, the stack must align with 16 byte */
 
-#define ARM64_CONTEXT_REGS  (37)
+#define ARM64_CONTEXT_REGS  (38)
 #define ARM64_CONTEXT_SIZE  (8 * ARM64_CONTEXT_REGS)
 
 #ifdef CONFIG_ARCH_FPU