servo_v4: update board_select_rp_value
servo_v4: update board_select_rp_value

This function sets the default Rp value, but should
not enable Rp if it's not already enabled.

BRANCH=servo
BUG=b:116630203
TEST=check CC lines after booting with/without charge through.

Change-Id: Ie9709796940cf908b9af3701d2123f16596f04e0
Signed-off-by: Nick Sanders <f4566bb3c90c0bf5fa0906b296b1da45051a62ee@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1244381
Reviewed-by: Aseda Aboagye <12c9b286316a940fd31c24070da5ab64cdd4d0e7@chromium.org>
Reviewed-by: Mary Ruthven <81380f3403fed74252e92df3ed012a1819ec6296@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec