opencl : alignment size converted from bits to bytes (#7090)

* opencl alignment size should be converted from bits to bytes

Reference: https://registry.khronos.org/OpenCL/specs/3.0-unified/html/OpenCL_API.html#CL_DEVICE_MEM_BASE_ADDR_ALIGN

> Alignment requirement (in bits) for sub-buffer offsets.

* Update ggml-opencl.cpp for readability using division instead of shift

Co-authored-by: Jared Van Bortel <cebtenzzre@gmail.com>

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Co-authored-by: Jared Van Bortel <cebtenzzre@gmail.com>