tools: ftdgrep: correct the find regions loop in do_fdtgrep

tools: ftdgrep: correct the find regions loop in do_fdtgrep

Use realloc and update the loop executed in do_fdtgrep to find all
the regions: only test count > max_region after the second pass.

This patch solve an issue if the number of region found (count)
is greater then the default value (max_region = count = 100):
the second pass is never executed, because the loop stops after
the first pass (i = 0, count > 100, max_regions = 100)
with error -1 and the error message
"Internal error with fdtgrep_find_region".

I also update the error message.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>