it8xxx2/linker: append h2ram_pool section at the end of used memory

Since __sha256_ram_block section must in the first 4KB,
h2ram_pool section is no longer included first inside the
RAMABLE_REGION.
Append h2ram_pool section at the end of used memory, so gap
due to alignment is still available for newly added variables.

Signed-off-by: Dino Li <Dino.Li@ite.com.tw>