touchpad_elan: Ensure we at least have 1k of shared memory
touchpad_elan: Ensure we at least have 1k of shared memory

Debugging commands may request buffers up to that size.

BRANCH=poppy
BUG=b:63993891
TEST=make buildall -j

Change-Id: I6dedfafc4e36d311026f9678e2cac99c85036ce0
Signed-off-by: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1071311
Reviewed-by: Vincent Palatin <70a9964ec8fd10b0b08fcc8623c2add25ddf99ac@chromium.org>

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