scripts: west_command: patch.py: fix sha256 bug on windows

Fix SHA256 verification failures on Windows by ensuring consistent
line ending handling in patch files. Replace inline hash calculation
with existing get_file_sha256sum() function and modify it to read
files as text with normalized line endings before encoding to UTF-8
for hashing.

This ensures cross-platform compatibility and prevents patch
integrity check failures due to CRLF/LF differences between
operating systems.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>