blk: Increase cache element size

Cache up to 4 kiB entries. 4 kiB is the default block size on ext4, yet
the underlying block layer devices usually report support for 512B . In
most cases, the 512B support is emulated (ie. SD cards, SSDs, USB sticks
etc.) and the real block size of those devices is much bigger.

To avoid performance degradation with such devices and FS setup, bump
the maximum cache entry size to 4 kiB.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Tom Rini <trini@konsulko.com>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>