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

Place serial 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
libdrivers__serial.a:   +2282 bytes      -226ms

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