nvmem: sunxi-sid: read NVMEM size from device compatible

Sometimes the SID device have more memory address space than the real
NVMEM size (for the registers used to read/write the SID).

Fetch the NVMEM size from device compatible, rather than the memory
address space's length, in order to prepare for adding some
registers-based read support.

Signed-off-by: Icenowy Zheng <icenowy@aosc.xyz>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>