soc: it8xxx2: linker: Move kernel library to RAM to enhance performance

Place kernel library in ILM. This can improve performance.

test:
Print the message 10000 times with 1ms sleep interval to compare the
time difference before and after adding RAM code on the it82002bw evb.

                         RAM code size    save time
original:                1954 bytes
libkernel.a:            +8941 bytes      -649ms

Signed-off-by: Tim Lin <tim2.lin@ite.corp-partner.google.com>