spi25: Extract 4BA preparations into new `spi25_prepare.c`

We will have more preparation steps for fast-read operations and
QPI in the future. Better start a new file, as `spi25.c` already
is rather long.

Change-Id: I253b270ce6796fb09e6d74903bd65a6fbc06c7d6
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.sourcearcade.org/c/flashprog/+/162
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
diff --git a/meson.build b/meson.build
index efe10cd..408b87a 100644
--- a/meson.build
+++ b/meson.build
@@ -69,6 +69,7 @@
   'programmer_table.c',
   'sfdp.c',
   'spi25.c',
+  'spi25_prepare.c',
   'spi25_statusreg.c',
   'spi95.c',
   'spi.c',