drivers: flashdisk: implement basic write caching

Cache written data to avoid rewriting same flash page multiple times
when writing subsequent flash pages. The cache is used for reads to
account for reading not yet committed (i.e. dirty) page data. Speeding
up reads is not intention of this patch and therefore the read path
does not modify cache state.

Fixes: #30212

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>