net/memac_phy: reuse driver for little endian SoCs

The memac for PHY management on little endian SoCs is similar on big
endian SoCs, so we modify the driver by using I/O accessor function to
handle the endianness, so the driver can be reused on little endian
SoCs, we introduce CONFIG_SYS_MEMAC_LITTLE_ENDIAN for little endian
SoCs, if the CONFIG_SYS_MEMAC_LITTLE_ENDIAN is defined, the I/O access
is little endian, if not, the I/O access is big endian. Move fsl_memac.h
out of powerpc include.

Signed-off-by: Shaohui Xie <Shaohui.Xie@freescale.com>
Signed-off-by: York Sun <yorksun@freescale.com>