MLK-12791 mx6qpsabresd: Change ENET TXCLK clock from PLL

MLK-12791 mx6qpsabresd: Change ENET TXCLK clock from PLL

In u-boot, i.MX6QP sabresd board uses 125Mhz ref clock from PHY,
While kernel uses the clock from internal PLL by setting GPR5 bit 9.
When doing warm reset in kernel, the GPR regigster is not reset, so
the clock source still is the PLL. This causes ENET in u-boot can't work.

In this patch, we change the u-boot to use internal PLL to align with
kernel for i.MX6QP. This also fixes the ENET issue after kernel warm reset.

Signed-off-by: Ye Li <ye.li@nxp.com>
(cherry picked from commit 7f00c72e17e4e440df62aa4945a619fdbc9cfd8f)