mx7ulp: Update unlock and refresh sequences in sWDOG driver

According to i.MX7ULP Reference Manual the second word write for both
UNLOCK and REFRESH operations must occur in maximum 16 bus clock.

The current code is using writel() function which has a DMB barrier to
order the memory access. The DMB between two words write may introduce
some delay in certain circumstance, causing a WDOG timeout due to 16 bus
clock window requirement.

Replace writel() function by __raw_writel() to achieve a faster memory
access and avoid such issue.

Reviewed-by: Ye Li <ye.li@nxp.com>
Signed-off-by: Breno Lima <breno.lima@nxp.com>