top: Add .editorconfig to set basic coding styles

EditorConfig [1] ensures consistent coding styles for developers
collaborating on the same project across various editors and IDEs. By
adopting the Linux project's .editorconfig settings [2] and applying
them to OpenSBI, it helps users maintain uniform indentation across
different IDEs and version control systems, improving development
efficiency.

The main difference from Linux project is the addition of
'trim_trailing_whitespace = true', which removes trailing whitespace
before newlines. This change affects only three files in OpenSBI, but
it improves overall code consistency.

 $ grep -nr "\s$" --include={*.{c,dts,h,lds,ldS,mk,s,S.py},Kconfig,Makefile,Makefile.*} .
 ./include/sbi_utils/fdt/fdt_fixup.h:4: * Implement platform specific DT fixups on top of libfdt.
 ./platform/generic/platform.c:79: * The fw_platform_coldboot_harts_init() function is called by fw_platform_init()
 ./platform/generic/platform.c:81: * according to the DT property "cold-boot-harts" in "/chosen/opensbi-config"

Link: https://editorconfig.org/ [1]
Link: https://lore.kernel.org/lkml/ZbAbmchIO8Cd5hNd@surfacebook.localdomain/T/ [2]
Signed-off-by: Hui Min Mina Chou <minachou@andestech.com>
Reviewed-by: Anup Patel <anup@brainfault.org>

.editorconfig

@@ -0,0 +1,21 @@
+# SPDX-License-Identifier: GPL-2.0-only
+# See here for more information about the format and editor support:
+# https://editorconfig.org/
+
+root = true
+
+[{*.{c,dts,h,lds,ldS,mk,s,S},Kconfig,Makefile,Makefile.*}]
+charset = utf-8
+end_of_line = lf
+trim_trailing_whitespace = true
+insert_final_newline = true
+indent_style = tab
+indent_size = 8
+
+[*.py]
+charset = utf-8
+end_of_line = lf
+trim_trailing_whitespace = true
+insert_final_newline = true
+indent_style = space
+indent_size = 4

.gitignore

@@ -5,6 +5,7 @@
 !.clang-format
 !.gitignore
 !.github
+!.editorconfig
 
 # Object files
 *.o