drivers: crypto: rsa_mod_exp: avoid DM_FLAG_PRE_RELOC

Function rsa_verify_key() is not called before relocation. So there is no
need to load the UCLASS_MOD_EXP drivers before relocation.

This avoid a failure to boot for pine64-lts_defconfig with CONFIG_RSA=y.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>