drivers: flash: spi_nor: make wait_until_ready erase delay configurable

The erase time varies between different SPI NOR flash chips.
Some have typical erase times in the 20-25ms range, at which point the
default 50ms poll interval means we get half the possible erase speed.
With slower memory, or larger erases, 50ms might not be a lot, but for
block erases, if we are unlucky we may end up polling just as the it's
about to finish erasing, and have to wait another poll interval.

Signed-off-by: Kamil Krzyzanowski <kamnxt@kamnxt.com>