board: rockchip: Fix RG353M model renaming

board: rockchip: Fix RG353M model renaming

Anbernic RG353M is hardware compatible with RG353P. Only the form-factor
differs. So only one DTS is created for both machines with
"Anbernic RG353P" as default model. If a RG353M is detected, the model
should be overwritten with the correct name.
Actually, it's overwritten with "Anbernic" only making the process of
machine detection a little harder.
However, to determine the size of the string "Anbernic RG353M", it is
sizeof() which is used resulting in obtaining the size of the pointer
(which is 8 bytes on ARM64) not the size of the pointed string.
strlen() should be used instead.

Signed-off-by: David Barbion <davidb@230ruedubac.fr>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>