gpio: dwapb: fix bgpio usage

The DW APB GPIO driver uses the generic GPIO library gpio-mmio,
and initialize the flags as "false", which should be 0.

When no flags are given, the native endianness is used to access
the MMIO registers, and the pin2mask() call can simply be
converted to a BIT() call, as per the default pin2mask()
implementation in gpio-mmio.c.

Acked-by: Alan Tull <atull@kernel.org>
Acked-by: Hoan Tran <hotran@apm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>