riscv: defconfig: enable NLS_ISO8859_1

riscv: defconfig: enable NLS_ISO8859_1

NLS_ISO8859_1 was enabled as a module with commit efe1e08bca9a ("riscv:
defconfig: enable NLS_CODEPAGE_437, NLS_ISO8859_1"), but the
NLS_CODEPAGE_437 counterpart is selected as built-in. The commit does
not explain the reason behind, and it is not consistent with the
defconfig for ARM64 that also enables these modules to mount EFI system
partitions.

Select NLS_ISO8859_1 as built-in to provide both requirements within the
kernel image.

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Link: https://patch.msgid.link/20260111-nls_iso8859_1_y_riscv-v1-1-2c992bb2c00d@gmail.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>