arch: riscv: common: linker: Add itcm & dtcm sections

The current riscv linker script don't have sections for
Data & Instruction Tightly Coupled Memory.  Add itcm and
dtcm sections to make it available.  All sections were
8 byte align to keep compatible with rv64 SoCs.

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