target/ppc: Restore [H]DEXCR to 64-bits

target/ppc: Restore [H]DEXCR to 64-bits

The DEXCR emulation was recently changed to a 32-bit register, possibly
because it does have a 32-bit read-only view. It is a full 64-bit
SPR though, so use the corresponding 64-bit write functions.

Fixes: fbda88f7abdee ("target/ppc: Fix width of some 32-bit SPRs")
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>