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: 17e11740 ("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>