power: reset: nvmem-reboot-mode: respect cell size for nvmem_cell_write

power: reset: nvmem-reboot-mode: respect cell size for nvmem_cell_write

Some platforms expose reboot mode cells that are smaller than an
unsigned int, in which cases lead to write failures. Read the cell
first to determine actual size and only write the number of bytes the
cell can hold.

Fixes: 7a78a7f7695b ("power: reset: nvmem-reboot-mode: use NVMEM as reboot mode write interface")
Signed-off-by: Alexander Koskovich <akoskovich@pm.me>
Link: https://patch.msgid.link/20251214191529.2470580-1-akoskovich@pm.me
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>