boards: nrf54h20pdk_nrf54h20: use XIP for PPR

The amount of RAM owned by PPR core is quite limited, making it
difficult to fit many samples. Instead, use execution in place and
increase its code partition from 28K to 64K (as now it doesn't have to
match RAM size).

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>