arm64/debug: Mask off all reserved bits from generated ESR values

There are only 16 comment bits in a BRK instruction, which
correspond to ESR bits 15:0.  Bits 24:16 of the ESR are RES0,
and might have weird meanings in the future.

This code inserts 16 bits of comment in the ESR value instead of
20 (almost certainly a typo in the original code).
Signed-off-by: NDave Martin <Dave.Martin@arm.com>
Acked-by: NMark Rutland <mark.rutland@arm.com>
Acked-by: NCatalin Marinas <catalin.marinas@arm.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>

arch/arm64/include/asm/debug-monitors.h

@@ -45,7 +45,7 @@
 /*
  * ESR values expected for dynamic and compile time BRK instruction
  */
-#define DBG_ESR_VAL_BRK(x)	(0xf2000000 | ((x) & 0xfffff))
+#define DBG_ESR_VAL_BRK(x)	(0xf2000000 | ((x) & 0xffff))
 
 /*
  * #imm16 values used for BRK instruction generation