keystone: init: enable UART1 to be able use it from kernel

Currently PWREMU_MGMT is not configured in the Linux generic UART
driver as this register seems to be specific TI UART IP. So this
needs to be enabled in u-boot to use UART1 from kernel space.

Acked-By: Vitaly Andrianov <vitalya@ti.com>
Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>