drivers: crypto: mod_exp_sw: Re-add DM_FLAG_PRE_RELOC

This driver is safe to use in SPL without relocation. Denying
DM_FLAG_PRE_RELOC prevents its usability for verifying the main U-Boot
or other artifacts from the SPL unless needless enabling the full driver
set (SPL_OF_PLATDATA).

Fixes: 17e1174085 ("drivers: crypto: rsa_mod_exp: avoid DM_FLAG_PRE_RELOC")
CC: Heinrich Schuchardt <xypron.glpk@gmx.de>
CC: Marek Vasut <marex@denx.de>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

drivers/crypto/rsa_mod_exp/mod_exp_sw.c

@@ -32,6 +32,7 @@ U_BOOT_DRIVER(mod_exp_sw) = {
 	.name	= "mod_exp_sw",
 	.id	= UCLASS_MOD_EXP,
 	.ops	= &mod_exp_ops_sw,
+	.flags	= DM_FLAG_PRE_RELOC,
 };
 
 U_BOOT_DEVICE(mod_exp_sw) = {