mtd: spi-nor-core: Consolidate non-uniform erase helpers for S25 and S28

s25_erase_non_uniform() and s28hx_t_erase_uniform() support hybrid sector
layout (32 x 4KB sectors overlaid at bottom address) and doing same thing.
Consolidate them into single helper named s25_s28_erase_non_uniform().

Signed-off-by: Takahiro Kuwano <Takahiro.Kuwano@infineon.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>