riscv: defconfig: enable NLS_CODEPAGE_437, NLS_ISO8859_1

The EFI system partition uses the FAT file system. Many distributions add
an entry in /etc/fstab for the ESP. We must ensure that mounting does not
fail.

The default code page for FAT is 437 (cf. CONFIG_FAT_DEFAULT_CODEPAGE).
The default IO character set is "iso8859-1" (cf. CONFIG_NLS_ISO8859_1).

So let's enable NLS_CODEPAGE_437 and NLS_ISO8859_1 in defconfig.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>