pnv/xive2: Add definition for TCTXT Config register

pnv/xive2: Add definition for TCTXT Config register

Add basic read/write support for the TCTXT Config register on P10. qemu
doesn't do anything with it yet, but it avoids logging a guest error
when skiboot configures the fused-core state:

qemu-system-ppc64 -machine powernv10 ... -d guest_errors
  ...
[    0.131670000,5] XIVE: [ IC 00  ] Initializing XIVE block ID 0...
XIVE[0] - TCTXT: invalid read @140
XIVE[0] - TCTXT: invalid write @140

Signed-off-by: Frederic Barrat <fbarrat@linux.ibm.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Message-Id: <20230601121331.487207-2-fbarrat@linux.ibm.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>