arm64/sysreg: improve comment for regs without fields

Currently for registers without fields we create a comment pointing at
the common definitions, e.g.

| #define REG_TTBR0_EL1                           S3_0_C2_C0_0
| #define SYS_TTBR0_EL1                           sys_reg(3, 0, 2, 0, 0)
| #define SYS_TTBR0_EL1_Op0                       3
| #define SYS_TTBR0_EL1_Op1                       0
| #define SYS_TTBR0_EL1_CRn                       2
| #define SYS_TTBR0_EL1_CRm                       0
| #define SYS_TTBR0_EL1_Op2                       0
|
| /* See TTBRx_EL1 */

It would be slightly nicer if the comment said what we should be looking
for, e.g.

| #define REG_TTBR0_EL1                           S3_0_C2_C0_0
| #define SYS_TTBR0_EL1                           sys_reg(3, 0, 2, 0, 0)
| #define SYS_TTBR0_EL1_Op0                       3
| #define SYS_TTBR0_EL1_Op1                       0
| #define SYS_TTBR0_EL1_CRn                       2
| #define SYS_TTBR0_EL1_CRm                       0
| #define SYS_TTBR0_EL1_Op2                       0
|
| /* For TTBR0_EL1 fields see TTBRx_EL1 */

Update the comment generation accordingly.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20220513174118.266966-2-mark.rutland@arm.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/tools/gen-sysreg.awk

@@ -184,7 +184,7 @@ END {
 	if (next_bit != 63)
 		fatal("Some fields already defined for " reg)
 
-	print "/* See " $2 " */"
+	print "/* For " reg " fields see " $2 " */"
 	print ""
 
         next_bit = 0