riscv: linker.ld: Fix undefined reference linker error

The commit a28830b8 aligned the data and rename some symbols.  However
there are two symbols at riscv linker script that were missing, which
causes below linker error:

kernel/xip.c:28: undefined reference to `__itcm_load_start'
kernel/xip.c:43: undefined reference to `__dtcm_data_load_start'

Rename below symbols to fix the issues.

__itcm_rom_start -> __itcm_load_start
__dtcm_data_rom_start -> __dtcm_data_load_start

Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>