target/arm: Correct encoding of Debug Communications Channel registers

target/arm: Correct encoding of Debug Communications Channel registers

We don't implement the Debug Communications Channel (DCC), but
we do attempt to provide dummy versions of its system registers
so that software that tries to access them doesn't fall over.

However, we got the tx/rx register definitions wrong. These
should be:

AArch32:
  DBGDTRTX   p14 0 c0 c5 0  (on writes)
  DBGDTRRX   p14 0 c0 c5 0  (on reads)

AArch64:
  DBGDTRTX_EL0  2 3 0 5 0 (on writes)
  DBGDTRRX_EL0  2 3 0 5 0 (on reads)
  DBGDTR_EL0    2 3 0 4 0 (reads and writes)

where DBGDTRTX and DBGDTRRX are effectively different names for the
same 32-bit register, which has tx behaviour on writes and rx
behaviour on reads.  The AArch64-only DBGDTR_EL0 is a 64-bit wide
register whose top and bottom halves map to the DBGDTRRX and DBGDTRTX
registers.

Currently we have just one cpreg struct, which:
 * calls itself DBGDTR_EL0
 * uses the DBGDTRTX_EL0/DBGDTRRX_EL0 encoding
 * is marked as ARM_CP_STATE_BOTH but has the wrong opc1
   value for AArch32
 * is implemented as RAZ/WI

Correct the encoding so:
 * we name the DBGDTRTX/DBGDTRRX register correctly
 * we split it into AA64 and AA32 versions so we can get the
   AA32 encoding right
 * we implement DBGDTR_EL0 at its correct encoding

Cc: qemu-stable@nongnu.org
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2986
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20250708141049.778361-1-peter.maydell@linaro.org