boards: arm: Reset Ethernet PHY after system timer on mimxrt1xxx_evk's

Moves the Ethernet PHY reset routine on mimxrt1xxx_evk boards to execute
in the POST_KERNEL initialization level rather than the PRE_KERNEL_2
level because it relies on a kernel service, k_busy_wait(). Leverages
the existing CONFIG_PHY_INIT_PRIORITY to allow users to configure when
within the POST_KERNEL level this routine occurs, which by default is
set correctly to the following order:

Init Function			Init Level	Priority
--------------------------------------------------------------------------------------
sys_clock_driver_init		PRE_KERNEL_2	CONFIG_SYSTEM_CLOCK_INIT_PRIORITY=0
mcux_igpio_##n##_init		POST_KERNEL	CONFIG_KERNEL_INIT_PRIORITY_DEFAULT=40
mimxrt1064_evk_phy_reset	POST_KERNEL	CONFIG_PHY_INIT_PRIORITY=70
eth_init			POST_KERNEL	CONFIG_ETH_INIT_PRIORITY=80
net_init			POST_KERNEL	CONFIG_NET_INIT_PRIO=90

This fixes an issue that was introduced in commit
f0b3146f, which caused the system timer
to be initialized after the Ethernet PHY reset. As a result, the
k_busy_wait() never returned and the kernel never finished booting. The
serial driver never initialized and there was no output on the console,
not even a boot banner.

Tested on the mimxrt1064_evk board with samples/net/dhcpv4_client.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>