hw/intc/arm_gicv3_its: Use FIELD macros for DTEs

hw/intc/arm_gicv3_its: Use FIELD macros for DTEs

Currently the ITS code that reads and writes DTEs uses open-coded
shift-and-mask to assemble the various fields into the 64-bit DTE
word.  The names of the macros used for mask and shift values are
also somewhat inconsistent, and don't follow our usual convention
that a MASK macro should specify the bits in their place in the word.
Replace all these with use of the FIELD macro.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>