doc: board: avoid ambiguous names for axy17lte

Model names are SM-A{3,5,7}20, just SM-{3,5,7}20 could also refer to
SM-J{3,5,7}20 or SM-T{3,5,7}20.

Fixes: 3e2095e9 ("board: samsung: add support for Galaxy A series
of 2017 (a5y17lte)")
Signed-off-by: Henrik Grimler <henrik@grimler.se>
Reviewed-by: Dzmitry Sankouski <dsankouski@gmail.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>